Abstract: Water to be treated within a water tank is circulated between the water tank and an electrolysis vessel by means of a circulating pump. In the electrolysis vessel, hypochlorous acid is generated by electrolysis with a pair of electrodes. The generated hypochlorous acid is introduced into the water tank and used for sterilization of the water to be treated. A first three-way valve is controlled to guide the water to be treated within the electrolysis vessel either to the water tank or to a drain. For a prescribed period of time after the polarities of the electrodes constituting the electrode pair are switched to each other, the water to be treated within the electrolysis vessel is guided to the drain to prevent impurities attached to the electrode surface from being introduced into the water tank.