To provide a low-cost hot water temperature controller for a water heater, allowing simplification of the hot water temperature controller and allowing reduction of the number of components by allowing suitable temperature control, boiling control and heat retention control by one hot water temperature detection sensor.

Water supplied from a water supply source is introduced to a lower part of a hot water tank 11, and is boiled by carrying electricity to a heater 19. An upper part of the hot water tank 11 is provided with a hot water takeout port 21, and a surface of the hot water tank 11 in a nearly same position as the hot water takeout port 21 is provided with the hot water temperature sensor 20. Suitable temperature decision is performed by the hot water temperature sensor 20, and hot water is taken out from the hot water takeout port 21. When water corresponding to a reduction amount by the taking-out of the hot water is introduced from the lower part of the hot water tank 11, reduction of hot water temperature by the introduction is transmitted to the hot water temperature sensor 20. When the hot water temperature sensor 20 detects that the hot water temperature decreases as compared to a set value, the heater 19 is supplied with current to boil the water, and the temperature retention control is performed.