An equal-parts blend of Kawaji Onsen's communal source and communal-bathhouse source, classified as a simple thermal spring. Weakly alkaline and only lightly mineralized, it is soft on the skin with very little bite, and a generous dose of metasilicic acid puts it in Japan's beauty-water family — the sort of silica film that leaves skin noticeably smoother afterwards. The posted table lists autonomic nervous system instability, insomnia and depressed mood as the spring-type-specific indications. Nothing is diluted here, but the water is reheated to a comfortable bathing temperature and chlorine-disinfected for hygiene; the same board notes that every bath except the roofed open-air lukewarm pool at Ojika-no-yu runs through a recirculation filter.
Good for
Smooth skin
Bicarbonate and alkalinity gently lift dead keratin while silica hydrates, leaving skin smooth. This is the basis of the beautifying-water (bijin-no-yu 美人の湯) reputation.
dry, rough skin
Recovery
Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.
fatigue, stress
Take care
- Active stage of any illness (especially with fever)
- Active tuberculosis
- Marked physical debilitation, such as advanced malignant tumor or severe anemia
- Severe heart or lung disease causing breathlessness on slight exertion
- Severe kidney disease with edema
- Gastrointestinal bleeding
- Visible bleeding
- Acute exacerbation of a chronic illness
Full composition
The bar is each ion's share of charge (mval%) within its group; over 20% names the spring. Gases, silica and trace metals are measured by mass only.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺75.9mgP22 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺17.2mgP35 national
- Potassiumminor; matters for drinking limitsK⁺4.4mgP32 national
- AmmoniumNH₄⁺0.50mgP48 national
- LithiumLi⁺0.20mgP38 national
- Magnesiummild; a laxative if you drink itMg²⁺0.20mgP15 national
- StrontiumSr²⁺0.10mgP24 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.0000mg
- BariumBa²⁺0.0000mg
- CopperCu²⁺0.0000mg
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.0000mg
- Iron (III)Fe³⁺0.0000mg
- Hydrogenmakes the water acidic — sharp and bactericidalH⁺0.0000mg
- Manganesetrace; companion to iron springsMn²⁺0.0000mg
- ZincZn²⁺0.0000mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻88.7mgP38 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻74.1mgP48 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻61.1mgP34 national
- Fluoridetrace; drinking limits applyF⁻3.4mgP69 national
- NitrateNO₃⁻1.7mgP90 national
- Bromidetrace, sea-originBr⁻0.40mgP41 national
- AsO2AsO₂⁻0.0000mg
- BO2BO₂⁻0.0000mg
- Carbonatestrongly alkaline; slippery feelCO₃²⁻0.0000mg
- H2PO4H₂PO₄⁻0.0000mg
- Hydrogen phosphateHPO₄²⁻0.0000mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻0.0000mg
- HSiO3HSiO₃⁻0.0000mg
- HSO4HSO40.0000mg
- Iodideyellows in air; helps blood lipidsI⁻0.0000mg
- HydroxideOH⁻0.0000mg
- SS²⁻0.0000mg
- Thiosulfateoxidized sulfurS₂O₃²⁻0.0000mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃65.0mgP40 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂7.0mgP45 national
- Metaarsenious acidHAsO₂0.10mgP53 national
- H2SO4FreeH2SO4Free0.0000mg
- Phosphoric acidH₃PO₄0.0000mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂6.0mgP31 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S0.0000mg
- ArsenicAs0.09mgP63 national
- CadmiumCd< —mg
- ChromiumCr< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
- Chronic muscle or joint pain and stiffness (rheumatoid arthritis, osteoarthritis, lower back pain, neuralgia, frozen shoulder, bruises, sprains — chronic phase)
- Muscle stiffness associated with motor paralysis
- Cold sensitivity
- Peripheral circulation disorders
- Reduced gastrointestinal function (heaviness, bloating)
- Mild hypertension
- Impaired glucose tolerance (diabetes)
- Mild hypercholesterolemia
- Mild asthma or emphysema
- Hemorrhoid pain
- Stress-related symptoms (sleep disorders, depressed mood)
Evidence
- Maeda M. 温泉の医学的効果とその科学的根拠. J. Hot Spring Science 70:197–207, 2021.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.