PURPOSE: To increase the heat exchange ratio of the subject device by detecting whether water is within a bathtub or not, and automatically supplying hot water to the bathtub when the temperature within a hot water storage tank reaches a set value.

CONSTITUTION: When the temperature of a heat collector 1 becomes higher than the temperature of water within a hot water storage tank 5, a circulation pump 4 is actuated, and the hot water is led into a heat medium heat exhanger 3 of high temperature. The heat is heat-exchanged with water within the tank 5. When the temperature within the tank 5 reaches a set value and it is confirmed by a detector 14 that no water is present within a bathtub 10, an electromagnetic valve 16 is opened, and high temperature water within the tank 5 is supplied to the bathtub 10. Although the temperature within the tank 5 is lowered by the supply water of low temperature, it is immediately heated by the heat collector 1.