To perish definitely bacteria existing in a circulation circuit of hot water in a bath which is continuously used for a long time.

There are provided a bath hot water supply heater 3, which is connected to a bathtub 4 and has a hot water supply function, a hot water feeding function and an additional heating function and a cleaner 1 provided in an additional heating circuit which connects the bathtub to the bath hot water supply heater. Additional heating operation is carried out until the hot water temperature reaches a specified temperature of 70° when a sterilization command is received and the sterilization is executed by maintaining the specified temperature for a specified time. Then water is poured into the bathtub 4 so that the hot water temperature may be lowered to a desired bathing temperature.