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In Japan, the definition of "hot spring" is established by Article 2 of the Hot Springs Act, enacted in 1948. This definition is primarily intended to distinguish it from ordinary water (such as well water) and is not necessarily based on the premise of bathing. Therefore, even if it qualifies as a "hot spring" under the Hot Springs Act, it does not necessarily mean it is a hot spring with expected bathing benefits.
There are facilities that have been cherished by locals as "mineral spring inns" for a long time, but since 1948, some places have not qualified as hot springs under the Hot Springs Act and can no longer display the hot spring sign.
The definition of "hot spring" according to Article 2 of the Hot Springs Act is as follows:
"In this law, 'hot spring' refers to hot water, mineral water, and water vapor or other gases (excluding natural gas with hydrocarbons as the main component) that have the temperature or substances listed in the attached table, which are discharged from underground."
Table 1: Attached Table of Article 2 of the Hot Springs Act (Rephrased for clarity)
| Temperature (Temperature when collected from the hot spring source) | 25℃ or higher |
|---|---|
| Substances (Any one of the following) | Content per 1kg |
| ・Total dissolved substances (excluding gaseous substances) | 1000mg or more |
| ・Free carbon dioxide (CO2) | 250mg or more |
| ・Lithium ion (Li＋) | 1mg or more |
| ・Strontium ion (Sr2＋) | 10mg or more |
| ・Barium ion (Ba2＋) | 5mg or more |
| ・Iron (II) ion or iron (III) ion (Fe2＋, Fe3＋) | 10mg or more |
| ・Manganese (II) ion (Mn2＋) | 10mg or more |
| ・Hydrogen ion (H＋) | 1mg or more |
| ・Bromide ion (Br－) | 5mg or more |
| ・Iodide ion (I－) | 1mg or more |
| ・Fluoride ion (F－) | 2mg or more |
| ・Hydroarsenate ion (HAsO42－) | 1.3mg or more |
| ・Metaarsenite (HAsO2) | 1mg or more |
| ・Total sulfur (S) (H2S＋HS－＋S2O32－) | 1mg or more |
| ・Metaboric acid (HBO2) | 5mg or more |
| ・Metasilicic acid (H2SiO3) | 50mg or more |
| ・Sodium bicarbonate (NaHCO3) | 340mg or more |
| ・Radon (Rn) | 20 (10 billionths of a curie unit) or more |
| ・Radium salt (Ra) | 1 ten-millionth of a mg or more |
According to this definition, "warm water" or "water with many components" that gushes from underground, "warm water vapor or gas," or "gas with many components" are considered hot springs, except for natural gas with hydrocarbons as the main component.
According to the attached table, if 1. the temperature when collected from the hot spring source is 25℃ or higher, 2. the dissolved substances excluding gas in 1kg of hot spring water are 1000mg (1g) or more, or 3. any one of the 18 specified unique components is present in the specified amount or more, it is recognized as a "hot spring."