PURPOSE: To easily wash away the dust at the bottom part of a hollow yarn membrane during backwashing by supporting and fixing the end part of a bundle of hollow yarns in a casing, placing the hollow yarn in the recessed part of a filter cartridge packed with activated carbon and protecting the hollow yarn with the contour of the cartridge.

CONSTITUTION: The end part 9a of the hollow yarn is inserted into a holding frame 10 and fixed with a synthetic resin. The obtained hollow yarn membrane filter is screwed into the rib 12 of a lid 3. The lid 3 is put on the upper part of the main body 1 and firmly attached so that a hollow yarn 9 is placed in the recessed part of the filter cartridge 5 packed with activated carbon 6 and wherein the recessed part is formed at the central part. When the hollow yarn is backwashed, the lid 3 is opened, the cartridge is released, the holding frame is released from the lid 3 and fitted into a rib 13 to direct the opening at the end face of the hollow yarn toward the water inlet side and then the lid 3 is put on the main body 1. Under such conditions, untreated water is supplied in the same way as in the ordinary operation and the dust deposited on the surface of the hollow yarn 9 is removed by the water current and discharged to the outside of the filter.