Kanto > Gunma > Kusatsu Onsen
100% natural free-flowing private baths all to yourself at an intimate six-room inn decorated with pressed flowers
Ekinariya Ryokan
益成屋旅館
406, Kusatsu-machi, Agatsuma-gun, Gunma 377-1711, JAPAN
Phone：+81-(0)279-88-2005
Spring Quality
Acidic, Sulfur containing ― Aluminum ―
Sulfate / Chloride springs
(Hydrogen sulfide type)
酸性・含硫黄－アルミニウム－硫酸塩・塩化物泉（硫化水素型）
(Hypotonic, Acidic, High temperature spring)
Four Distinct Types in One Naturally Free-Flowing Onsen /
Kusatsu’s Premier Hot Spring for Therapeutic Benefits and Beauty
Hot springs are believed to form when rainwater or snowmelt seeps underground, or when ancient seawater becomes trapped deep within the earth. Heated by magma or the earthʼs internal heat, this water dissolves minerals from underground rocks and slowly rises to the surface over years—or even tens of thousands of years.
Onsen Components
(Shirahata hot spring source)
* % values are simplified representations of mval%.
9.3 mg/kg
35.48 mval%
■ The main component of Acidic springs (pH 2.0), which are believed to support insulin function and help improve impaired glucose tolerance (diabetes)
■ The main component of “Hot springs for skin conditions”, helping to suppress acne and eczema through its powerful bactericidal effect
■ Provides a natural peeling effect by dissolving and removing old dead skin cells
■ Creates an acidic environment that helps tighten the skin
■ Stimulates skin renewal and promotes healthy cell turnover
■ More than 9 times
the standard value for acidic springs
47.1 mg/kg
20.04 mval%
■ Provides astringent action that tightens the skin and pores
■ Leaves the skin feeling firm and refreshed after bathing
■ Helps suppress exudate (oozing discharge) from inflamed or irritated skin
■ Provides antiperspirantaction
*Supporting Component
18.5 mg/kg
■ Just 1.5 mg/kg more would qualify it for classification as Iron springs (20 mg/kg or more)
■ With an additional 1.5 mg/kg, it would serve as one of the characteristic constituents of historic Ryokuban-sen (Vitriol springs), once known for strong bactericidal properties and referred to as “Hot springs for skin conditions” (modern classification: Iron–Aluminum–Sulfate springs)
■ Exhibits a potent astringent (skin-tightening) effect through synergistic interaction with aluminum and sulfate ions
■ Oxidizes into iron(III) upon exposure to air, contributing to changes in water color
* % values are simplified representations of mval%.
685.5 mg/kg
54.32 mval%
■ Supports peripheral blood circulation
■ Promotes skin metabolism, helping improve firmness and elasticity
■ Aids recovery from skin conditions such as dry skin and cuts
■ Contributes to autonomic nervous system regulation through warming and circulation-promoting effects
■ The main component of Sulfate springs, known as “Hot springs for stroke prevention” for their circulation-promoting effects, “Hot springs for cuts” for their bactericidal and anti-inflammatory properties, and “Hot springs for beauty and skin revitalization”
328.1 mg/kg
35.22 mval%
■ Supports recovery from cuts through the bactericidal action of salts
■ Enhances heat retention by forming a salt coating on the skin surface
■ Helps retain skin moisture by reducing water evaporation through the salt-coating effect
■ Contributes to autonomic nervous system regulation through sustained warming effects
■ The main component of Chloride springs, known as “Hot springs for warmth” for their long-lasting warming effects and “Hot springs for cuts” for their bactericidal properties
*Supporting Component
10.0 mg/kg
■ 5 times
the hot spring standard (2 mg/kg or more)■ Bactericidal and antimicrobial action: Helps suppress the proliferation of microorganisms on the skin surface
■ Helps soothe skin irritation: May help relieve acne, eczema, and mild inflammatory symptoms
* The bars for Metasilicic acid 50 mg/kg and Metaboric acid 5 mg/kg represent the qualification criteria for hot springs. The number of bars indicates values such as “more than XX times the hot spring standard” or “about XX times the hot spring standard”.
Metasilicic Acid 240.6 mg/kg■ Natural moisturizing component
■ More than 4.8 times the hot spring standard (50 mg/kg or more)
Metaboric Acid 9.6 mg/kg■ Anti-acne and purifying effects
■ About 1.9 times the hot spring standard (5 mg/kg or more)
■ The component responsible for the characteristic “rotten egg” odor of Sulfur springs
■ The main component of “Hot springs for skin conditions”, helping to relieve symptoms of skin disorders through its powerful antibacterial and anti-inflammatory properties
■ Hydrogen sulfide is absorbed through the skin, dilating peripheral blood vessels and improving circulation
■ The main component of “Inhalation springs” and “Hot springs to help clear phlegm”, helping to ease asthma symptoms and improve mucus clearance
■ Helps remove excess sebum and is beneficial for acne-prone and oily skin
■ The main component of “Hot springs for beautiful skin and prevention of dark spots”, helping to suppress melanin production and promote skin-brightening effects
■ Through its reducing action, sulfur reacts with reactive oxygen species associated with aging, with potential anti-aging effects
■ Contains about 3 times
the standard threshold required for classification as a sulfur spring (2 mg/kg or more of total sulfur) (Total sulfur: 6.02 mg/kg)
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Total Dissolved Substances
(excluding gases)
1,784 mg/kg
12～18 times more components than commercial bath additives!
Hydrogen Ion
Concentration
pH 2.0
Acidic
Spring Temperature
51.9 ℃
■ Temperature adjusted by flow rate
Color of water /
Smell of water
Immediately after discharge
Almost clear and colorless
After some time
Milky white
Sulfur smell like rotten eggs
Hot Spring Flow Rate
20.42 L/min
(Shared Source / Naturally Flowing)
Hot spring water usage per person (spring discharge / maximum capacity)
→ 20.42L / 16 guests ＝ 1.27 L / per minute
* To meet the requirements for kakenagashi (flowing directly from the source),
at least 1 liter of hot spring water per minute per bather
is required for proper bath management
Status of Source Utilization
| Water added | Heated | Disinfected | Circulated | Bath additives |
|---|---|---|---|---|
| No | No | No | No | No |
* 100% natural free-flowing hot spring water: None of the baths use circulation, filtration, or disinfection.
Large Public Bath (for men) “Benten-no-Yu”
The luxurious hinoki cypress bath is the eye-catching centerpiece of the Large Public Bath “Benten-no-Yu”. With only six guest rooms and private baths available for exclusive use, the public bath rarely feels crowded even when the inn is fully booked—a luxury in itself.
Skin Feeling Gauge
Ideal for:
People with skin conditions such as atopic dermatitis
People with lifestyle-related conditions (e.g. diabetes)
People with minor cuts
People seeking skin‑beautifying effects
People concerned about dark spots
People seeking firmer, more radiant skin
People with dry skin
People sensitive to cold
People feeling stressed
Indications by Spring Quality
(for bathing)
Plaque psoriasis
Chronic eczema
Atopic dermatitis
Epidermal suppuration
Impaired glucose tolerance (diabetes)
Cuts
Peripheral circulatory disturbance
Sensitivity to cold
Depression
Dry skin
General Indications (for bathing)
Chronic pains or stiffness of muscles or joints (chronic phase of rheumatoid arthritis, osteoarthritis, lower back pains, nerve pains, frozen shoulder, bruises, sprains, etc.), Muscle stiffness in motor paralysis, Sensitivity to cold, Peripheral circulatory disturbance, Gastrointestinal hypofunction (slow digestion, intestinal gas formation, etc.) Mild hypertension, Impaired glucose tolerance (diabetes), Mild hypercholesterolemia, Mild asthma or Emphysema, Hemorrhoidal pains, Autonomic instability, Various symptoms from stress (sleep disorders, depression, etc.), Restorative phase from illness, Recovery from fatigue, Health promotions
General Contraindications (for bathing)
Active stage of diseases (especially when fever is accompanied), Active tuberculosis, Advanced
malignant tumor or the case of significant debility involving severe anemia, Severe cardiac or lung
diseases involving suffocation feelings with a little movement, Severe kidney diseases involving
edema, Gastrointestinal bleeding, When bleeding is visible, Acute exacerbation stage of chronic
diseases
*There are no general indications or contraindications for drinking.
Contraindications by spring quality (for bathing)
Hypersensitive skin or mucous membranes, Dry skin in the elderly
Onsen Power Chart
→ 100% natural free-flowing hot spring water
All baths are supplied with 100% natural free-flowing hot spring water
Hot Spring Beauty Benefits Chart
Large Public Bath “Benten-no-Yu” Hot Spring Outlet
The men's and women's large public baths “Benten-no-Yu” draw their water from the Shirahata source, one of Kusatsu Onsen's most renowned hot spring sources. The milky emerald-green, 100% natural free-flowing hot spring water offers an exquisite bathing experience.
Keywords of this Onsen
This hot spring is believed to work in the following three stages:
STEP １
Bactericidal & Reset Zone
(Cleanse the skin through bactericidal action)
Acidic spring (Hydrogen ions) with bactericidal and peeling effects / Aluminum ions that help tighten the skin and pores / Iron-containing spring (Iron ions), known as “Hot springs for women” / Chloride spring (Chloride ions), known as “Hot springs for cuts” for its bactericidal action of salts
STEP ２
Hot Springs for Skin Conditions Zone
(Soothe skin troubles and inflammation)
Acidic spring (Hydrogen ions) with a powerful bactericidal effect / Sulfur spring (Free hydrogen sulfide) with antibacterial and anti-inflammatory properties / Iron-containing spring (Iron ions), known as “Hot springs for women" / Sulfate spring (Sulfate ions), known as “Hot springs for cuts" due to its anti-inflammatory effects
STEP ３
Hot Springs for Beautiful Skin Zone
(Beautify and refine the skin)
Sulfur spring (Free hydrogen sulfide) known for its skin-brightening and dark spot prevention benefits / Sulfate spring (Sulfate ions) associated with wrinkle prevention and skin revitalization / Calcium ions that strengthen the skin's barrier function / Metasilicic acid with hydrating and moisturizing effects / Chloride spring (Chloride ions), known for its salt-coating and moisturizing effects
■ Indications by Spring Quality (for Bathing)
| Acidic, Sulfur containing ― Aluminum ― Sulfate / Chloride springs (Hydrogen sulfide type) |
Corresponding Spring Quality | |||
|---|---|---|---|---|
| Acidic Spring | Sulfur Spring | Sulfate Spring | Chloride Spring | |
| Atopic dermatitis | ○ | ○ | ||
| Plaque psoriasis | ○ | ○ | ||
| Impaired glucose tolerance (diabetes) | ○ | |||
| Chronic eczema | ○ | |||
| Epidermal suppuration | ○ | ○ | ||
| Peripheral circulatory disturbance | ○ | ○ | ○ | |
| Cuts | ○ | ○ | ||
| Sensitivity to cold | ○ | ○ | ||
| Depression | ○ | ○ | ||
| Dry skin | ○ | ○ | ||
■ Contraindications by Spring Quality (for Bathing)
| Acidic, Sulfur containing ― Aluminum ― Sulfate / Chloride springs (Hydrogen sulfide type) |
Corresponding Spring Quality | |||
|---|---|---|---|---|
| Acidic Spring | Sulfur Spring | Sulfate Spring | Chloride Spring | |
| Hypersensitive skin or mucous membranes | ○ | ○ | ||
| Dry skin in the elderly | ○ | ○ | ||
Private Bath “Ruri-no-Yu”
There are six private baths, one for each of the inn's six guest rooms. Two of them, “Ruri-no-Yu” and “Chiyo-no-Yu”, draw their water from the Shirahata source. Here, guests can enjoy the renowned hot spring water all to themselves.
Onsen Classification
️Hot springs are classified not only by their spring quality, but also by three additional factors: osmotic pressure, hydrogen ion concentration (pH level), and spring temperature.
Osmotic Pressure
(Hypotonic, Acidic, High temperature spring)
In hot spring classification, “osmotic pressure” refers to comparing the concentration of dissolved components in the spring water (mainly salts) with that of human body fluids (such as blood and intracellular fluid). Based on this comparison, springs are categorized as hypotonic, isotonic, or hypertonic. The difference in concentration serves as an indicator of how easily water moves between the hot spring water and body fluids.
Total Dissolved Substances
1,784 mg/kg
Classified as a Hypotonic spring
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Isotonic 👤 Body fluid concentration ≒ ♨ Onsen concentration Water・・・Hardly moves into or out of the body Onsen components・・・Minimally absorbed and act gently on the body Isotonic means having the same osmotic pressure as human body fluids. It is equivalent to a saline solution made by dissolving approximately 8.8 g of salt in 1 liter of water. Because its concentration is close to that of body fluids, there is little movement of water or onsen components due to osmotic pressure. As a result, it is considered to place the least strain on the body and to be the gentlest in terms of stimulation. |
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Hypertonic 👤 Body fluid concentration ＜ ♨ Onsen concentration Water・・・Easily moves out of the body Onsen components・・・Easily absorbed into the body Because its osmotic pressure is higher than that of human body fluids (isotonic fluids), water tends to move out of the body more easily, while onsen components are more readily absorbed into the body. As a result, stronger therapeutic effects may be expected. However, hypertonic hot springs are also more likely to cause “hot spring fatigue (yu-atari)” and can be more physically demanding. They are generally regarded as “rich or highly concentrated hot springs”. Among Japanese hot springs, hypertonic springs are relatively uncommon, and a comparatively large proportion are classified as mineral springs (below 25°C) rather than hot springs (25°C or higher). |
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Hydrogen Ion Concentration
(Hypotonic, Acidic, High temperature spring)
In the hot spring analysis report, the section titled “Examination and test results at gushing location” includes the listed pH value. Chemically, a pH of 7.0 is considered neutral; values below this are acidic, and values above it are alkaline. According to the “Guidelines for the Mineral Spring Analysis Methods”, the chemical nature of a mineral spring is classified based on its pH value at the time of emergence, as shown in the table below.
pH 2.0
Classified as Acidic
Spring Temperature
(Hypotonic, Acidic, High temperature spring)
51.9℃
Classified as High temperature spring
Under the “Hot Springs Act”, water that contains one or more designated substances at or above the legally specified levels and has a source temperature of 25°C or higher is classified as a “hot spring”. If the temperature is below 25°C, it is labeled a “mineral spring”. Furthermore, as noted above, hot springs are classified by source temperature as high-temperature spring, hot spring, or low-temperature spring.
Private Bath “Seki-no-Yu”
The four private baths, “Seki-no-Yu”, “Netsu-no-Yu”, “Washi-no-Yu”, and “Matsu-no-Yu”, use hot spring water from the Yubatake source. Though small, these private hot spring baths offer the rare luxury of bathing whenever you like without worrying about others.
Summary
The Shirahata hot spring source used by Ekinariya Ryokan in Kusatsu Onsen is highly acidic and rich in sulfur, combining the qualities of four spring types: Acidic, Sulfur, Sulfate, and Chloride springs.
Acidic and Sulfur springs offer excellent bactericidal properties that help with various skin concerns, while Sulfur springs are also expected to provide skin-beautifying effects by gently removing keratin and sebum. Sulfate springs interact with aluminum ions to tighten the skin and pores and also promote skin revitalization, earning them, together with Sulfur springs, the name “Hot springs for beautiful skin”. Chloride springs are known as “Hot springs for warmth” for their strong heat-retention effects.
The water is also rich in metasilicic acid, a component known for its moisturizing properties.
This milky-white, naturally free-flowing hot spring (“gensen kakenagashi”), combining exceptional therapeutic and skin-beautifying components, truly deserves to be called one of Japan’s premier hot springs.
Facility Data
| Founded in | 1969 (Showa 44) |
|---|---|
| Check in | 2:00 PM |
| Check out | 10:00 AM |
| Location | Mountain / Town |
| Rooms | All 6 rooms (6 Japanese-style rooms) |
| Capacity | 16 persons |
| Parking | Available for 6 cars |
| Facilities | Shop |
| Internet | Wi-Fi available |
| Barrier-free | Not supported |
| Bidet toilet | Equipped in all guest rooms |
Hot Spring (Onsen) Report
| This inn draws from the Shirahata source and the Yubatake source, two of the representative spring sources of Kusatsu Onsen. Both spring qualities are “Acidic, Sulfur containing ― Aluminum ― Sulfate / Chloride springs (Hydrogen sulfide type)”. In Japan, hot springs are classified into 10 types, of which “Acidic spring”, “Sulfur spring”, “Sulfate spring”, and “Chloride spring”, which means this spring has the characteristics of 4 different types of hot springs. First, one of the conditions for “Acidic spring” is that 1 mg or more of hydrogen ions must be contained in 1 kg of hot spring water. The pH value (hydrogen ion concentration) of this hot spring is 2.0, which is acidic, and is often shows strong acidity. With its extremely high antibacterial power, it is effective against atopic dermatitis, trichomonas vaginalis, and athlete's foot, and is called “Hot springs for skin diseases” because it widely removes the causes of problems on the surface of the skin. Also, the indications by spring quality include Plaque psoriasis, Impaired glucose tolerance (diabetes), and Epidermal suppuration. It is curious that the color of the hot spring water right after it gushes out from the source is clear and colorless, but it appears ・・・ Read more → |
Transportation Access
| By train | JR Ueno station, Joetsu, Azuma line, New limited express Kusatsu → JR Nagano Kusatsuguchi station in about 3 hours → JR bus for Kusatsu Onsen → about 25 min., JR Kusatsu Onsen bus terminal → 5 min. by walk |
|---|---|
| By bus | JR highway bus Jyoshu Yumeguri-go at Basta Shinjuku → about 4 hours, JR Kusatsu Onsen bus terminal → 5 min. by walk |
| By car | Kanetsu Expressway, Nerima IC → about 3 hours, exit at Shibukawa-Ikaho IC → Use Route 17, 353, 145, or 292 |
| Address | Gunma／Kusatsu Onsen／Ekinariya Ryokan 406, Kusatsu-machi, Agatsuma-gun, Gunma 377-1711, JAPAN TEL:+81-(0)279-88-2005 |