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What Is a Hot Spring Analysis Certificate? | Legal Requirements & How to Read It

Published May 6, 2026·1min read

Every licensed onsen facility in Japan is required by law to display an official hot spring analysis certificate (温泉分析書). It documents the spring's chemistry, pH, spring type, and health indications — here's what to look for.

Table of Contents
  1. Legal Basis for the Certificate
  2. Key Items Listed on the Certificate
  3. What to Focus On

Q & A

Frequently Asked Questions

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Where is the analysis certificate displayed?
It is typically posted in the changing room or near the entrance to the bathing area. If you cannot find it, ask the front desk staff.
Is it a problem if the certificate is old?
The Hot Spring Act requires renewal every ten years. A certificate more than ten years old is overdue for renewal. However, if there have been no significant changes to the spring, the fundamental characteristics remain valid.

Under Article 18 of Japan's Hot Spring Act, any facility offering hot spring water for bathing or drinking must display the analysis certificate inside the bathing area. Analysis must be conducted by a prefectural government-registered body and renewed every ten years.

Key Items Listed on the Certificate

  • Spring name, location, and analysis date
  • Flow rate, source temperature, and pH
  • Major components (cations, anions, undissociated components, dissolved gases) in mg/L
  • Official spring type (with classification basis)
  • Bathing and drinking indications and contraindications
  • Bathing notes: whether water is heated, diluted, circulated, or disinfected

What to Focus On

  • pH: low = acidic, high = alkaline; affects skin stimulation
  • Metasilicic acid & metaboric acid: trace components associated with skin-softening benefits
  • Free carbon dioxide: ≥ 2,000 mg/L qualifies as a carbon dioxide spring
  • Heating / dilution notes: key to identifying "source-flow" (kakenagarashi) springs
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References

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