PURPOSE: To ozonize water in accordance with the inflow rate of water by controlling the flow rate of waste ozone to fix the water level in an ozone contact tank and injecting ozone in such an amt. that the ozone absorption is optimized.

CONSTITUTION: The water level in an ozone contact tank 11 is fixed by controlling the flow rate of waste ozone through a controller 34, a flowmeter 31 and a valve 27 in accordance with the inflow rate of water to the tank 11 detected by a flowmeter 20. Meanwhile, the injection rate of ozone is controlled so that the ozone absorption obtained from the ozone concn. by an ozone densitiometer 15 and the waste ozone concn. by a waste ozone densitiometer 26 is controlled within specified limits. The fluorescence intensity of treated water is allowed to conform to the treated water quality by the detection values from fluorophotometers 21 and 33, the desired injection rate of ozone is obtained so that the fluorescence intensity is controlled within specified limits, and the ozone injection rate and the dissolved ozone concn. detected by a densitiometer 32 are controlled.