PURPOSE: To decompose org. matters contained in waste water by oxidation with high efficiency and to prevent generation of sludge by performing oxidation treatment of waste water filled in a reaction apparatus by dispersing ozonized gas in the waste water simultaneously with electrolysis and irradiation with ultraviolet rays of the waste water.

CONSTITUTION: After regulating concn. and flow rate of ozonized gas by an ozone generator 15 and a flow rate controller 14, the gas is dispersed in the water to be treated through a gas dispersing device 13 having fine pores, and rises upward while keeping contact with the water to be treated and discharged into atmosphere after passing through an exhaust port 16 and a gas absorption tower 17. At the treatment, fixed amt. of raw water to be treated is filled in the reaction tank through an inlet 2 of the raw water at the top of the reaction tank, and samples of treated water are collected appropriately from sampling port 4 while circulating a part of the treated water through circulation passage provided with a circulating pump 6. The treatment is performed while grasping the quality of the treated water, and the treated water is discharged out of an outlet port 3 at the bottom of the reaction tank.