To use water in a water storing tank also as an electrolytic water and to eliminate supply of ion exchange water by a method wherein ozone gas formed by electrolyzing water which has passed through an ion exchange resin tower from a water storing tank in an anode room is fed into the water storing tank from a circulation water tank and it is mixed with water in the water storing tank to clarify the water storing tank.

A water storing tank 1 is connected with pollution-preventive apparatus 2 through a communicating pipe 3, and an electrolytic cell 5 and an ion exchange resin tower 4 are made to communicate with each other from a water inlet 21 of the ion exchange resin tower 4 to the bottom opening part 33 of an anode room of the electrolytic cell 5. A circulation tank 6 is made to communicate with the top opening part 40 of an anode room of the electrolytic cell 5 and is made to communicate with the bottom opening part 33 through a return pipe 41. A mixed gas of oxygen gas and ozone gas generated in the anode room is transferred to the circulation tank 6 through a gas transferring pipe 39. One of outlets 42 through which the mixed gas is exhausted is connected with a water drawing pipe 46 of a mixing pump 45 through a fine pipe 44 and the other one is made to communicate with the open air from an atmospheric pressure control opening part through an ozone decomposition catalyst in a pressure balancer 48.