To provide a filter material allowing free adjustment of the magnitude and area of the opening of the material lapped in the filtration chamber.

In the filter material to be stretched in the filtration chamber of a separating apparatus, fixed filter materials 3 and 4 and sliding filter materials 5 and 6 are lapped, and the opening ratio of the pores 1a and 2a of the filter material is adjustable by sliding the sliding filter materials 5 and 6. Since the area of the openings 7 and 8 of the fixed materials 3 and 4 and the sliding materials 5 and 6 overlapped one upon another can be changed arbitrarily in the range of 0-100%, an appropriate area of the opening is obtainable according to the nature of activated sludge, eliminating the need of replacement of the filter material.

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