PURPOSE: To eliminate discharge of offensive odor into a room and to prevent cooling of a bath heat exchanger by starting counting of a timer after a fan for the exchanger is stopped, and operating a fan for supplying hot water for a predetermined period of time.

CONSTITUTION: When an additional combustion of a bathtub 12 is ended, a burner 2 is stopped, a fan 3 for a bath heat exchanger is stopped, and counting of a timer for measuring a time after the end is started. If it is not during heating of a hot water supplying heat exchanger 4, it is regarded for a fan 6 to be driven for reignition after hot water supply is used, but if the fan 6 is not during operating, the fan 6 is turned ON for a predetermined time to discharge offensive odor due to heating of a connecting part of the fan 3 to an exhaust connection cylinder 7, etc., out of a room, thereby avoiding a decrease in a temperature of bathtub water.