Abstract: Primarily a pair of substantially fan-shaped filter sheets are laid one upon another and bonded to each other at least in the proximity of their side edges to obtain a basic filter 4 adapted to form a bag having a circular opening as arc-shaped outer edges 24A, 24B of these filter sheets, wherein a pair of substantially crescent-shaped flap sheets 6 are bonded along their arc-shaped edges to said basic filter 4 along their arc-shaped outer edges 24A, 24B to obtain a filter 1 provided with a pair of pockets 8 openable downwardly of the filter 1.