PURPOSE: To eliminate trouble in low cost, by forming a filter device, an ozone generator for injecting ozone in filtered water and a magnetic water treatment device to a water purifying apparatus.

CONSTITUTION: Cooling water is taken out of a cooling tower 1 in a flow rate 5W40% of the recirculation flow rate of a main flow passage 8 to be divided by a pair of the branch pipes 6a, 6b of a recirculation passage 6. A part of the cooling water flows in one branch pipe 6a in a flow rate 1W10% of the recirculation flow rate of the main flow passage 8 to be filtered by a jet strainer 3. Then, the SS component and org. substances or algae consuming ozone in the cooling water are removed. Subsequently, the cooling water is mixed with ozonated air in a diffuser 13 and a mixed stream flows in an ozone reaction chamber 4 to receive sterilization reaction by ozone. By this method, sterilization can be efficiently performed and hard scale can be removed.