PURPOSE: To suppress foaming that arises in the initial period of an operation by providing a diffuser which feeds ozone in the form of foam into waste water in the lower part in a vessel and a shower device which sprays the waste water near the diffuser onto the surface of the waste water in the upper part of the vessel.

CONSTITUTION: Waste water is introduced through an introducing pipe 2 into an ozone reaction vessel 1 and the treated waste water is discharged through a release pipe 8. The ozone generated in an ozonizer 10 is introduced through an introducing pipe 4 into a diffuser 5 which is provided in the lower part of the vessel 1 and is formed of a porous material. The ozone is fed through such diffuser into the waste water. The treated water having a high concn. of the dissolved ozone in the lower part of the tank 1 is sucked through a pipeline 12 and is lifted by a pump 13 onto the water surface and is sprayed onto the foam on the water surface from a shower 14. The antifoaming is thus efficiently accomplished, by which the height of the foam is decreased and the decrease in the ozone decomposing capacity in an ozone decomposition tank 11 is obviated.