To provide a water heater capable of effectively using hot water or a heating medium at intermediate temperature within a tank.

When detection temperature by a thermistor TH1 exceeds hot water filling set temperature+10C, an auxiliary heat source device 4 and a reheating heat source device 41 are not operated, and hot water is heated by the heating medium led out from an upper part of a hot water storage tank 2 to fill hot water at set temperature in a bathtub 90. When detection temperature by the thermistor TH1 is below the hot water filling set temperature+10C and exceeds average water supply temperature to a heat exchanger 3 for hot water supply+10C, while the operation of the auxiliary heat source device 4 is prohibited, hot water is heated by the heating medium led out from the upper part of the hot water storage tank 2 to fill hot water at low temperature in the bathtub 90. After the filling of the hot water, the reheating heat source device 41 is operated to make the hot water within the bathtub 90 at the hot water filling set temperature.