PURPOSE: To effectively prevent the generation of the biological system trouble of a drain ditch without using a chemical agent and generating secontary pollution by continuously performing operation of blowing ozonized oxygen in waste water collected from the drain ditch to mix the same with the waste water and subjecting this waste water to ozone oxidation treatment to return the treated water to the drain ditch.

CONSTITUTION: A part of waste water is collected from a drain ditch by a device 1 and ozonized oxygen generated from an ozone generator 2 is blown in and mixed with the collected waste water using a device 3 to obtain ozonized oxygen-dissolved water. This obtained ozonized oxygen-dissolved water is mixed with the collected waste water in an ozone oxidation treatment tank 4 to be used in the ozone oxidation treatment of waste water. The treated water after ozone oxidation treatment is returned to the drain ditch. Ozonized oxygen is dissolved in this treated water and the waste water of the drain ditch is subjected to sterilizing and algicidal treatment even by this treated water to be gradually purified. These operations are continuously performed.