PURPOSE: To raise cleaning efficiency, by sending at least part of circulating water to an ozone treating tank to change a COD and a TOC components to a BOD component for easy organism treatment.

CONSTITUTION: Circulating water to be fed to a culture tank 2 is partially oxidized and decomposed under an aerobic condition in an aerobically cleaning tank 12a and partially denitrogenated in an anaerobically cleaning tank (denitrogenating device) 12b. Further the circulating water is partially treated by oxidizing action in an ozone treating tank 13, organic substances are made into low molecular substances to be decomposable by organism treatment. The circulating water is returned to the aerobically cleaning tank 12a again and subjected to cleaning treatment by oxidizing decomposition under an aerobic condition.