To realize the mixture treatment for obtaining a water quality not higher than those determined by the water quality standard and a control target water quality.

Substances not to have been decomposed by an ozone treatment are removed out of treated water by an active carbon contact basin 21 and highly purified treated water is flowed out. Existing treated water of water quality level lower compared with the level of the highly purified treated water is mixed in a mixing section 22, and the mixed flow rate water is intrudes into a mixture treatment tank 24 through a flow rate control valve 23. The water temperature and the UV value of the existing treated water are measured by a first measurement section 25, and the water temperature and the UV value of the active carbon contact basin 21 and conductivity thereof are measured by a second measurement section 26 while the water temperature and the UV value of pure water flowed out of the mixture treatment tank 24 and residual chlorine thereof are measured by a third measurement section 27, and based on the respective measured values, the water quality is predicted by a mixing system water quality predicting section 28, and predictions are fed into a mixing treatment control section 29 to find out the mixing ratio. Based on the mixing ratio thus found out, the mixing flow rate is found, and based on the flow rate, a control signal of a flow rate control valve 23 is issued from the control section 29.