Into an electrolytic bath storing water to be treated, saturated sodium chloride solution identified as an electrolysis accelerator is added from an electrolysis accelerator bath. The water to be treated is subjected to electrolysis. In the electrolytic bath, water to be treated is introduced from an inlet during the electrolysis process. The water subjected to electrolysis in electrolytic bath is output to a reservoir via an overflow port. A plurality of electrode pairs are provided in the electrolytic bath. During the electrolysis process, the value of the current flowing across the electrodes of the electrode pair arranged most upstream of a water channel from the inlet to the overflow port in the electrolytic bath is detected, and control is provided such that the current value is within a predetermined range by adjusting the concentration of the accelerator in the electrolytic bath.