The law: 25 °C, or the right minerals

Japan's "onsen" is defined by the Hot Spring Act of 1948. Water or steam that rises from the ground counts as an onsen if it meets either of two conditions:

  • it is 25 °C or warmer at the point where it emerges, or
  • it contains at least a set amount of any one of 19 listed substances (total dissolved solids, sulfur, iron, radon and so on).

So cold water can be an onsen if it carries enough minerals — these are the "cold mineral springs" (reikosen), such as Masutomi in Yamanashi. And water with almost no minerals is still an onsen if it is 25 °C or more.

The 25 °C line was chosen because it is clearly warmer than ordinary groundwater in Japan.

"Therapeutic springs" are a stricter class

Among onsen, those whose minerals exceed a higher threshold are called "therapeutic springs" (ryoyosen). Only therapeutic springs receive a spring-type name such as simple hot spring, sulfur spring or radioactive spring. Water that is legally an onsen but falls short of these levels has no spring-type name.

The criteria are a temperature of 25 °C or more, or one of seven substances (total dissolved solids, free carbon dioxide, total iron, hydrogen ions, iodide, total sulfur, radon) above its limit. This is why some pages in the atlas say "spring type unconfirmed" or "below therapeutic thresholds".

Natural onsen and other baths

City bathhouses and large "super sento" may use legal onsen water, heated tap water, or bath additives that mimic an onsen ("artificial onsen"). The way to tell is the notice in the changing room: facilities using onsen water must post the water analysis and state whether water is added, heated, recirculated or disinfected.

In this atlas, facilities whose natural-spring status we could not confirm from sources are marked "natural spring unconfirmed", so they are not mistaken for onsen.

Onsen do not come only from volcanoes

Volcanic springs such as Kusatsu and Noboribetsu are famous, but onsen also exist far from any volcano. Underground, the temperature rises by roughly 3 °C every 100 metres, so a well deeper than about 1,000 metres yields water above 25 °C in much of Japan. Tokyo's black "kuroyu" and Chiba's iodine springs are non-volcanic onsen carrying ancient seawater or plant-derived compounds.

The difference shows in the water: volcanic springs tend to be acidic or sulfurous, while non-volcanic springs are often salty, bicarbonate-rich or iodine-bearing.

Kawa-no-ie inn at Yoro-keikoku Onsen 白蛇の騎士 · CC BY-SA 3.0

Yōrō-keikoku Onsen

養老渓谷温泉

Chiba · 100 Onsen to Know Japan — No. 36

ChlorideBicarbonate

Dark-brown water in a Boso gorge where there are no volcanoes: the spring comes up cold, around 20°C, and is heated for the bath.

Hotels along the Tokachi River 663highland · CC BY 2.5

Tokachigawa Onsen

十勝川温泉

Hokkaido · 100 Onsen to Know Japan — No. 6

ChlorideBicarbonate

Not volcanic heat but ancient plants: the brown, silky 'moor' spring that defines the type in Japan.

Glossary

Hot Spring Act (Onsen-ho)
Enacted 1948. Defines onsen, licenses drilling and requires notices for bathers. Administered by the Ministry of the Environment.
Therapeutic spring (ryoyosen)
An onsen meeting the temperature or seven-substance criteria, and therefore given a spring-type name.
Cold mineral spring (reikosen)
Water below 25 °C that qualifies as an onsen by its minerals. Usually heated for bathing.

Related onsen

Steaming Yubatake with wooden channels Aspere · CC0

Kusatsu Onsen

草津温泉

Gunma · 100 Onsen to Know Japan — No. 25

AcidicSulfurSulfateChloride

Thirty-two thousand litres a minute of pH 2.1 acid water rise in the middle of town and cool as they run down wooden channels.

Masutomi Onsen

増富温泉

Yamanashi · 100 Onsen to Know Japan — No. 44

Radon

Thirty quiet minutes in water around 30 °C. A therapeutic village where one of Japan's most radon-rich springs is bathed in unheated, exactly as it comes.