To provide a water treatment method hard to generate the clogging of a filter member and capable of easily washing the filter member to restore water treatment capacity.

Water 11 to be filtered is stored in an aeration tank 82 and the filter member 40 constituted by gathering string-like fiber aggregates is dipped in the water to be filtered. The water to be filtered is filtered at a place where it enters the gaps between fibers from the whole surface dipped in water of the filter member and small groups of microorganisms incapable of entering the gaps between fibers adhere to the surfaces of fibers and the water to be filtered flows through the gaps between fibers naturally by a capillary phenomenon. Thereafter, the small groups of microorganisms adhering to the surfaces of fibers become large gradually as filtering advances to form microorganism colonies. Subsequently, the water to be filtered is filtered at a place where it enters the minute gaps in the microorganism colonies and naturally flows through the minute gaps by a capillary phenomenon to subsequently flow through the gaps between fibers by a capillary phenomenon. The filtered water 25 is discharged to the outside of the aeration tank from the filter member.

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