To solve a problem that deterioration of white water quality and paper breaking during paper making which happen in reusing pulp.

In reusing pulp included in a supernatant by separating white water into the supernatant and pulp in a settling tank 1, ozone is injected from an ozonizer 4 into the supernatant coming from the settling tank 1 to oxidize the supernatant in an ozone retaining tank 3 and subsequently the supernatant is sent to the following process. Ozonization of the white water sent to the settling tank is also included. The injected amount of ozone in an ozone treatment device is controlled in a feedback control or a feed forward control by using the pulp concentration, the viscosity of treated water and the concentration of discharged ozone as controlling indicators.