To conduct volume filtration by making it difficult for a clad to intrude into a filter, to efficiently filter off the iron clad without precoating it and to remove the iron clad with a large throughput for a long time.

A cylindrical filter medium 3 consisting of a porous membrane is compressed in the axial direction, the filter medium 3 between the circular mountain parts 8 is pressed inward and folded to form a circular main fold 11 is the shape of a bellows, the small auxiliary fold 12 formed inside is folded back, and the main folds 11 are brought into contact with each other close to the outer periphery to form a main gap 13. A liq. to be treated is made pass through a filter 1 with an auxiliary gap 14 formed between the auxiliary folds 12 from the outside to remove an iron clad.