A filter bag (1) comprises a first thin plastic film (22); a second thin plastic film (21) having substantially the same size with the first plastic film and sealed to the first plastic film at a first side-peripheral edge (2a), a second side-peripheral edge (2b) opposite to the first side-peripheral edge and a bottom peripheral edge (2d) thereof to provide an open-top interior, and a filter sheet (5) having a length substantially equal to the first and second plastic films and a width smaller than the first and second plastic films. The filter sheet has a first longitudinal edge (5a) interposed and sealed together between the first side-peripheral edges of the first and second plastic films and a second longitudinal edge (5b), opposite to the first longitudinal edge, sealed to the inside of the first plastic film at an intermediate position in width of the filter bag. The filter sheet is also sealed to the inside of the second plastic film, in vicinity to the first longitudinal edge, to provide a half-length seam (5c) extending from the open top edge halfway toward the bottom of the filter bag. A sample receiving section (3) of a larger volume is defined between the first and second plastic films and the filter sheet whereas a suspension collection section (4) of a smaller volume is defined between the first plastic film and the filter sheet. The first plastic film preferably includes a pull-tab (7) near the open top edge, between the second longitudinal edge and the half-length seam. The pull-tab is manually pulled down over a given length to provide a pipette-inserting window.