To provide a device capable of supplying radon gas of the most important required condition in response to requirement, because a hospital in the city requires clearly a facility for coping with a problem of severe location, a full reservation state in reception allowance and the like, and a situation incapable of coping with the serious condition of a disease, in aspect of a situation of a hot-spring care site.

This radon gas generator sets a condition of powdering a radioactive natural rock to expand extremely a contact area with a gas phase, and generates the radon gas of required level by circulating the gas phase, in an independent cabinet of 40-60°C having ≥70% of humidity, using a water soluble condition of the radon gas.