To improve the washing force of flushing water by providing an electric heater heating the water in a water storage tank in the tank storing washing water for a flush toilet and heating the washing water in accordance with the temperature of the washing water in the water storage tank.

A water storage tank 2 for washing water W supplied to a toilet stool 1 is attached to the toilet stool 1. Tap water is supplied to the water storage tank 2 so that it is always filled with a constant quantity of tap water. An electric heater 22 is arranged in the water storage tank 2 and connected to a bimetal 23 in series through an electric source cord 3. When the washing water W gets cold, the bimetal 23 is closed and the washing water W is heated by the electric heater 22. When the washing water gets to a specified temperature, the circuit is disconnected by the bimetal 23 to stop electric supply to the heater 22. A water level sensor is attached to the water storage tank 2 so as not to supply electric power to the heater 22 regardless of the opened/closed condition of the bimetal 23 in the state the electric heater 22 is not submerged in the washing water W.