To facilitate maintenance by preventing adhesion of scales to electrodes.

An electrolytic sterilizer for a water tank comprises a bypass line made by connecting a suction pipe and a delivery pipe to a water tank, and connecting a circulating pump to the other ends of the suction and delivery pipes, a first electrolytic cell 7 disposed in the middle of the suction pipe or of the delivery pipe, a first power supply circuit connected to the electrodes of the first electrolytic cell 7 to supply a current to both electrodes while periodically inverting the polarity of the current, a hypochlorous acid tank 11 for storing a solution prepared by diluting hypochlorous acid liquid with tap water or the like, a hypochlorous acid feed line disposed between the hypochlorous acid tank 11 and the middle of the delivery pipe to send the hypochlorous acid of the hypochlorous acid tank 11 to the delivery pipe, a hypochlorous acid feed pump 12 connected to the middle of the hypochlorous acid feed line, a second electrolytic cell 13 disposed in the middle of the hypochlorous acid feed line, and a second power supply circuit connected to the electrodes of the second electrolytic cell to supply a current to both electrodes while periodically inverting the polarity of the current.