To provide a water heater applicable to an ordinary bathtub requiring no hot water reservoir in the bottom face or an existing bathtub.

A suction pipe 5e is extended from a single port circulation port 5 to the vicinity of the bottom face of a bathtub 2, a self-suction circulation pump 12 is disposed in the way of a return pipe 9 from the circulation port 5 to a heat exchanger 13 for heating bath water, a first change-over valve 16 is provided in the way of a going pipe 15 from the heat exchanger 13 to the bathtub 2, one outlet of the valve 16 is coupled with a second change-over valve 17, the other outlet of the valve 16 is coupled with a cleaning nozzle 20, one outlet of the valve 17 is coupled with the circulation port 5 and the other outlet of the valve 17 serves as a drain pipe 18.