To provide a bath water heater capable of detecting whether or not such a leaked state that hot water in a hot water storage tank and bath water in a circulating circuit is mixed occurs in the disposed part of the circulating circuit in the hot water storage tank even if the bath water heater comprises the circulating circuit partly disposed in the hot water storage tank.

A control device 200 closes motor valves 12 and 51 and, after stopping a circulating pump 11, determines whether or not the detected value of a pressure sensor 52 is 50 kPa or less. When the detected value exceeds 50 kPa, a heating part 10c as the disposed part of the circulating circuit 10 in the hot water storage tank is detected to be in leaked state.