To provide a small-sized device by which stable concd. ozonized water with the ozone concn. variable is obtained with high ozone dissolving efficiency.

This device is provided with an ozonizer 4, a water feed pump 6 for sending the water to be treated, a reaction tank 1 for reacting the ozone gas generated from the ozonizer with the water to be treated, a liq. discharge valve 12 for adjusting the discharge of the water to be treated, a pressure control valve 9 for controlling the ozone gas pressure and a waste ozone decomposing tank 10 for decomposing the reacted ozone gas. The reaction tank is formed with a high-pressure vessel, and a holding means is provided to keep the ozone gas pressure and water level independently constant. The holding means is furnished with a pressure sensor 13 for detecting the pressure of the ozone gas in the reaction tank, a level gage 14 for measuring the level of the water to be treated and a control unit 15 for processing the data from the pressure sensor and level gage and sending a driving command to the driving parts 8 and 11 of the pressure control valve and liq. discharge valve.