PROBLEM TO BE SOLVED: To suppress a water circulatory system applied to fish breeding water tank or the like from becoming larger and a maintenance cost from increasing.SOLUTION: A water treatment system includes filtration treatment parts (3, 5) disposed in a water circulatory path (CM) and biological treatment parts (7, 9) for biologically treating cleaning waste water generated by cleaning of the filtration treatment part. According to this, the biological treatment is applied not to a total amount of water in a circulatory system containing a water storage part such as a water tank but to only a part thereof, that is, reverse washing waste water. Further, when constituting such that the cleaning waste water is anaerobically treated (7), followed by aerobically treating (9) and by returning a part of aerobically treated water to the anaerobically treating part, in the anaerobically treating part, by utilizing an organic substance contained in the introduced cleaning waste water, denitration of nitric acid generated by the aerobic treatment can be performed. Therefore, an addition of methanol or the like can be made unnecessary.SELECTED DRAWING: Figure 1
