Onsen OniOnsen Oni

Search Onsen Oni

Search onsen, areas, prefectures and pages.

Sansuikan

Water profile

Sansuikan

Spring chemistry and what it does, with the research behind it.

花の里温泉 一号泉Hana no Sato Onsen Spring No. 1

湯Simple

The first of two wells the inn drilled on its own grounds, this radon-bearing alkaline simple thermal water is the one piped to the large indoor bath. Low in mineral bite and gentle on the body, it still carries the slippery, soft-skin feel that comes with a pH of 8.8 — squarely in Japan's beauty-water family. The panel documents it for convalescence, fatigue recovery and stress relief, and for neuralgia and joint stiffness; as a drinking spring it lists chronic digestive complaints, constipation and hangover, while noting that the water standing in the bath itself is not for drinking. The house adds that this source makes particularly good coffee.

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

On the certificate:Fatigue recoveryGeneral health improvement

Measured at the source

Source temperature
28.5°C
Low temp.
Tonicity
Hypotonic

On the certificate

  • Convalescence
  • Stress relief
  • Neuralgia
  • Muscle pain
  • Joint pain
  • Frozen shoulder
  • Motor paralysis
  • Joint stiffness
  • Bruises
  • Sprains
  • Chronic digestive disorders
  • Hemorrhoids
  • Chronic digestive disorders
  • Diabetes
  • Hangover
  • Constipation

Evidence

  1. 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.
  2. Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
  3. Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
  4. Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.

花の里温泉 弐号泉Hana no Sato Onsen Spring No. 2

曹Bicarbonate

The inn's second well feeds the open-air bath: a sodium bicarbonate spring, the classic jūsō-sen style. At pH 9.2 it is firmly alkaline, and the poster puts the bicarbonate content at 850 mg/kg — the combination behind its smooth, slippery beauty-water finish. It is documented for cuts, burns, chronic dermatitis, cold sensitivity and chronic gynecological conditions, and as a drinking spring for chronic digestive complaints, diabetes and liver conditions, though the panel flags that the water standing in the bath is not drinkable. The house notes this source makes excellent onsen yudofu (tofu simmered in hot-spring water).

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

Aches & joints

Warmth eases stiffness and raises the pain threshold. Radon and sulfate springs show the strongest evidence for joint and muscle pain.

stiff shoulders

On the certificate:NeuralgiaMuscle painJoint painFrozen shoulderMotor paralysis

Recovery

Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.

fatigue, stress

On the certificate:Frailty in childrenFatigue recoveryGeneral health improvement

Measured at the source

Source temperature
42.0°C
Hot

On the certificate

  • Cuts
  • Burns
  • Chronic dermatitis
  • Chronic gynecological conditions
  • Chronic digestive disorders
  • Hemorrhoids
  • Cold sensitivity
  • Chronic digestive disorders
  • Diabetes
  • Liver disorders
  • Gout

Evidence

  1. 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.
  2. Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
  3. Donaubauer AJ et al. Serial radon spa therapy on pain in musculoskeletal disorders. Frontiers in Immunology, 2024.
  4. Verhagen AP et al. Balneotherapy for rheumatoid arthritis. Cochrane Database of Systematic Reviews CD000518, 2015.
  5. Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
  6. Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.

Educational, not medical advice. If you have a health condition, consult a physician before onsen therapy.