PURPOSE: To realize a further exact discharged ozone density constant oontrol by adopting a manipulated variable fixing control at the time of both activation and fluctuation, and further quickly and stably obtaining a control target value.

CONSTITUTION: Ozone air generated by an ozone generdtor 1 is injected through an air diffusing tube 2 to an ozone processing water tank 3, and a discharged ozone density measured by a discharged ozone densitometer 4 is inputted to an ozone injection control circuit 5. Then, a voltage control signal to a high frequency invertor power source 6 of the ozone generator 1 is computed by the ozone injection control circuit 5, and outputted, so that the discharged ozone density can be turned to the preliminarily set discharged ozone density. The algorithm of the arithmetic operation is largely divided into three parts. That is, the arithmetic operation is performed firstly according as the ozone generator is activated (control at the time of activation), secondly according as a difference between the preliminarily set discharged ozone density and the discharged ozone density measured by the discharged ozone densitometer is within a certain range (normal control), and thirdly according as the difference between the preliminarily set discharged ozone density and the discharged ozone density measured by the discharged ozone densitometer is beyond a certain range (control at the time of fluctuation).