PURPOSE: To rapidly supply water high in the achievable degree of clarification of water quality at a low cost by filtering suspended matter in raw water to separate and remove it, blowing ozonized oxygen into the filtered water to mix it therewith and continuously performing operation for returning the obtained treated water to raw water.

CONSTITUTION: Water of an impounding reservoir is drawn up by a circulation pump 1. Refuse and suspended solid mixed in raw water are removed by a continuous filter 2. Ozonized oxygen is blown into the filtrate and mixed therewith in a device 4. Water dissolved with ozonized oxygen generated from an ozonizer 3 is obtained in the device 4. Successively oxidizing treatment with ozone is performed in an ozone oxidizing treatment device 5. Oxidizing treatment of the filtrate is completed and thereafter the filtrate is returned to the original impounding reservoir by a circulation pump 6. Therein the ozonized oxygen is obtained by utilizing oxygen generated from an oxygen PSA device 3 as a raw material and ozonizing it. In such a way, clarification of water quality of a pond in a park and a golf course and water in the impounding reservoir of the various factories is rapidly performed at a low cost without using a chemical.