A bathing apparatus for thermotherapy includes a bathtub provided with sensors for measuring a water temperature in the bathtub and for measuring the body temperature of a bather. A water supply/heating device adjusts the temperature of water by heating the water in a boiler based on the difference in temperatures measured by the sensors and circulates the water through a circulating passage leading to the bathtub. An oxidation-reduction potential reducing unit is provided in the circulating passage. It includes a container through which water in the circulating passage passes, and vitreous inorganic particles stored in the container for reducing the oxidation-reduction potential of water in the circulating passage when brought into contact with the water in the circulating passage. With this arrangement, because the oxidation-reduction potential of the water in the bathtub is reduced by the oxidation-reduction potential reducing unit, it is possible to reduce the water temperature felt by the bather by at least 1 to 2° C. if the actual water temperature is e.g. 43° C. The patient can thus be subjected to thermotherapy for a long period of time with reduced heat stress.