To provide a simple electrode cleaning mechanism capable of cleaning the surface of an internal electrode without disassembling an electrolytic treatment tank and capable of cleaning the electrode without stopping a drain water treatment apparatus even during the operation of the drain water treatment apparatus.

In the electrolytic treatment tank having a double structure composed of an almost cylindrical cathode also used as a part of a container and an almost cylindrical anode provided inside the cathode so as to leave a definite interval, a cleaning instrument having the annular brush coming into contact with the surfaces of both electrodes and the rodlike handle piercing through the opening provided to the container to protrude to the outside are provided to slide the cleaning instrument by operating the handle.