Every guide to Japan explains the difference between an onsen and a sento in more or less the same way: one is a hot spring, the other is a public bath. That is true, and it makes it sound like a matter of feel.
It is not. Onsen is a legal category with a statutory definition, a numeric threshold, and a list of minerals in an appended table. And the law requires the bathhouse to post the analysis where you can read it — which means you can check, in the changing room, before you get in.
The definition, and the word that changes everything
The act defines onsen as water, mineral water, steam or other gas emerging from underground — expressly excluding natural gas whose main component is hydrocarbon — which has the temperature or the substances set out in the appended table.
That single conjunction is why the popular understanding of onsen is slightly wrong. Water can fail the temperature test completely and still be an onsen, on the strength of what is dissolved in it.
Twenty-five degrees, measured at the source
The first limb of the table is temperature, and it sets 25 degrees Celsius, measured as the temperature when the water is drawn from the source. That is far below bathing temperature, which tells you something the brochures do not: a great many onsen are heated before they reach the bath, and heating does not disqualify anything. The test happens at the source.
Or one mineral, in a listed amount
The second limb is a list of substances, and the requirement is any one of them, at or above a stated amount per kilogram of water. These are the ones the table states:
| Substance | Amount required, per kilogram |
|---|---|
| Dissolved substances (excluding gaseous) | 1,000 mg in total |
| Free carbon dioxide | 250 mg |
| Lithium ion | 1 mg |
| Strontium ion | 10 mg |
| Barium ion | 5 mg |
| Ferrous or ferric ion | 10 mg |
| Manganese(II) ion | 10 mg |
| Hydrogen ion | 1 mg |
| Bromide ion | 5 mg |
| Iodide ion | 1 mg |
| Fluoride ion | 2 mg |
The table continues past these, and we are stopping where our reading of the original text is certain. What matters is the structure, not a complete inventory: one qualifying substance is enough, and there are many ways to qualify.
The sign in the changing room is a legal document
This is the part worth carrying with you. Anyone who provides onsen water for public bathing or drinking must display specified information in a conspicuous place inside the facility, in the manner ministry ordinance requires — including the cautions that apply to bathing in it or drinking it.
You have almost certainly walked past one. It is the framed panel of numbers and chemical names, usually near the lockers, that looks like a certificate nobody reads. It is posted because the act says it must be.
And the numbers are not the operator's own claim
The act closes that loop too. The posted content must be based on an analysis carried out by an organization registered with the prefectural governor for the purpose of performing hot spring analyses. So the figures come from a body that had to apply, meet requirements about its facility and its instruments, and be entered on a register.
There is a 30-day clock on keeping it current
An operator must undergo re-analysis at intervals fixed by cabinet order, and then — this is the specific bit — within 30 days of being notified of the result, must change the posted content in line with it. We are not telling you what the interval is, because that is in a cabinet order we did not retrieve. The 30-day deadline is in the act itself.
And a governor can order the wording changed
Before posting the notice, or changing what it says, the operator must notify the prefectural governor in advance. And where it is necessary to protect the health of people bathing in or drinking the water, the governor may order the content of that notice to be changed. That is a supervisory power over the sign itself, which is not something most visitors would guess is sitting behind a panel on a changing-room wall.
How to actually use this on a trip
The practical version is short, and it is more reliable than reading the name over the door.
What the panel will not settle
It will not tell you whether the water is diluted, reheated, recirculated or treated. Those disclosures sit in ministry ordinance detail we did not retrieve for this guide, so we are not going to describe how they are labeled. It also will not tell you anything about the classification language you may have seen — simple springs, therapeutic springs and so on — because that is a separate framework, not the appended table.
And it will not tell you about tattoos
Nothing in this act touches that question. Whether a particular bath admits guests with tattoos is a decision by the operator, and the picture is more mixed than most visitors expect. We cover what the government has actually said to the industry in our guide to tattoos and onsen.
Why the water is protected before it reaches you
The act is not primarily a consumer-protection statute. Its stated purpose is to protect hot springs, to prevent disasters from flammable natural gas released during extraction, and to ensure appropriate use — in that order. Most of its length deals with the source rather than the bath.
Drilling requires permission
A person intending to drill land for the purpose of causing onsen water to emerge must apply to the prefectural governor and obtain permission, and must hold the right to use that land for drilling. This is not a formality: the act lists specific grounds for refusal.
| The governor may refuse where… | What it protects |
|---|---|
| The drilling would affect the yield, temperature or composition of the hot spring | Existing sources, and the town built around them |
| The facility or method does not meet the technical standards for preventing gas disasters | Safety during extraction |
| The drilling would harm the public interest | A general catch-all |
| The applicant has a recent conviction or revoked permission under the act | Repeat offenders |
The first ground is the interesting one for a traveler. It means that in a hot spring town, a new hotel cannot simply drill its own well if doing so would draw down what the existing baths depend on. The old inns at Ginzan or Kusatsu are sitting on a resource the statute treats as shared and finite.
What this changes about how you choose a bath
Honestly? For most trips, very little — and that is worth saying plainly rather than pretending otherwise. The bath that makes your evening will make it whether or not the water clears a mineral threshold.
What it changes is the vocabulary. Once you know that onsen is a defined term with a posted certificate behind it, a lot of Japanese travel writing becomes readable in a way it was not before, and the distinctions inns draw between their baths stop looking like marketing. When a ryokan makes a point of its source, it is pointing at something with paperwork behind it.
Why a 1948 statute is doing this job
The act dates from 1948, and reading it with that in mind explains its shape. It was written in a period when the country was working out how to hold on to resources it could not replace, and the drafting reflects that: the source is protected first, the extraction is licensed second, and the person in the bath appears late, as someone entitled to be told what they are sitting in. The consumer-facing part of the act is the smallest part of it, which is why the notice on the wall reads like a technical document rather than a marketing panel. It was never designed to sell you anything.
It also explains why the thresholds look arbitrary. They are not a description of what feels like a hot spring — they are a line drawn so that a specific set of waters could be identified, registered and protected. Twenty-five degrees is not a promise about your bath. It is a boundary marker on a resource.
The one habit worth forming
Read the panel once, at the first onsen of your trip. Not to verify anything — just so you know what it looks like. After that you will notice its presence or absence everywhere, without effort, for the rest of your time in Japan. It is the same kind of small literacy as knowing which side of a road to walk on when there is no sidewalk: useless until the moment it quietly is not.
Deliberately not stated here
The act was our only source. These did not come from it, so they are gaps rather than estimates:
| What we are not saying | Why |
|---|---|
| How often re-analysis is required | Set by cabinet order, which we did not retrieve. Only the 30-day deadline is in the act. |
| How dilution, heating or recirculation must be disclosed | Ministry ordinance detail, not retrieved. |
| The spring-type classifications (simple, sulfur, therapeutic and so on) | A separate framework from the appended table. Not covered by what we read. |
| The complete list of qualifying substances | Part of the original table did not render reliably. We list only what we could read with certainty. |
| Penalty amounts | The penalty articles were not retrieved. |
| Rules for sento (ordinary public baths) | A different statute, not retrieved. |
| Whether a given bath admits tattoos | Not governed by this act at all. |
Onsen law FAQ
Is onsen a legal term in Japan?
What temperature makes water an onsen?
Can cold water be an onsen?
What is the sign on the wall of the changing room?
How current does that analysis have to be?
Can a bathhouse write whatever it likes on that sign?
Does calling a place a hot spring mean it is one?
Is drilling for a hot spring regulated?
Bottom line (2026) — onsen is a statutory category, not a mood. The Hot Springs Act defines it as underground water, mineral water, steam or gas with the temperature or the substances in its appended table — 25°C at the source, or any one listed mineral at its stated amount, which is why a cold spring can qualify. Anyone supplying that water for public bathing must post the analysis inside the facility, based on a registered analyst's work, updated within 30 days of a new result, filed with the prefecture, and changeable on the governor's order. So: read the panel in the changing room. It is the answer to the question.
Sources
This guide was written from the Hot Springs Act (Act No. 125 of 1948) and its appended table, retrieved in Japanese from the Japanese government legal database on September 1, 2026: the statutory definition of onsen as underground water, mineral water, steam or other gas excluding hydrocarbon-based natural gas, having the temperature or the substances in the appended table; the table's temperature limb of 25 degrees Celsius at the source and its list of qualifying substances and minimum amounts per kilogram; the duty to display specified information in a conspicuous place inside the facility for anyone supplying onsen water for public bathing or drinking; the requirement that the display be based on an analysis by a registered organization; the duty to undergo re-analysis at intervals set by cabinet order and to update the display within 30 days of notification; the duty to notify the prefectural governor in advance and the governor's power to order the content changed; the permission requirement for drilling and the listed grounds for refusal, including effects on the yield, temperature or composition of a hot spring; and the act's stated purpose of protecting hot springs and preventing gas disasters. Every value the act delegates to a cabinet order or ministry ordinance is flagged above rather than filled in. More on how we work: how we research.
Laws change. This reflects the text as it stood on September 1, 2026.
in English
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