PURPOSE: To increase the capacity of a filter by constituting a lattice of a pair of parallel longitudinal members one of which engages with the folded parts of the pleats of filter materials and the other films a pair of strips and plural traversing strut members having flat (round) surfaces.

CONSTITUTION: The stringers 11 on the rear side of separators 10 are formed to a cylindrical or sectional circular shape and the edges 11a thereof are engaged with the inner folded parts of the pleats of the filter materials. The plural struts 13 have the flat (round) front and rear surfaces and are joined at one end with the stringers 11 and are joined at the other end with the longitudinal members 13, 14 in a front side, by which the strips 15 are formed. The pleats of the filter materials are formed by drawing out filter material sheets 16 downward from rolls. While the separators 10 fold back the filter materials, the stringers 11 come into contact with the filter materials. Positioning is thus executed. The steps described above are repeated until the desired pleats are formed. The resulted assembly is compressed and is inserted into a frame 17.