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Hot springs are defined by a law called the "Hot Springs Law",
as hot water, mineral water, and steam or other gases that emerge from the ground,
● with a temperature of 25°C or higher when collected from the source or,
● containing one or more of 19 specified substances in a certain amount
and are all referred to as "hot springs".

Therefore, even if the temperature is below 25°C, groundwater containing, for example, a certain amount of carbon dioxide is called a "hot spring". Of course, if the temperature is 25°C or higher when it emerges, it is undoubtedly called a hot spring. In addition to this definition, there is a regulation called the "Guideline for Mineral Spring Analysis" set by the Ministry of the Environment, which provides for "therapeutic springs" (hot springs expected to have effects for the purpose of treating diseases), and hot springs that meet the criteria for therapeutic springs are given a spring quality name.

Definition of "Hot Spring" under the Hot Springs Law

1. Temperature: 25°C or higher (temperature when collected from the hot spring source).
2. Substances: (Must contain at least one of the following substances.)

Total dissolved substances (excluding gaseous substances)	1,000mg or more	Fluoride ion	2mg or more
Free carbon dioxide	250mg or more	Hydroarsenate ion	1.3mg or more
Lithium ion	1mg or more	Metaarsenite	1mg or more
Strontium ion	10mg or more	Total sulfur	1mg or more
Barium ion	5mg or more	Metaboric acid	5mg or more
Ferro or ferric ion	10mg or more	Metasilicic acid	50mg or more
Manganese ion	10mg or more	Sodium bicarbonate	340mg or more
Hydrogen ion	1mg or more	Radon	20×10-10Ci or more
Bromide ion	5mg or more	Radium salt	1/100,000,000mg or more
Iodide ion	1mg or more

(Content per kilogram)