PURPOSE: To prevent dispersion in filter performance and to prevent clogging by folding a thin flat plate in a lot of planes and constituting a filter element so that a lot of triangular gaps formed by covers closing a valley of a pleat may by uniform.

CONSTITUTION: A flat plate is folded to provide pleats so that the directions of the folds are alternately reversed, and one fold 1 of the pleat is given plastic deformation to closely adhere surfaces facing each other forming the pleat to each other at a vertex 1 of the pleat, and the other vertex 2 of the pleat is bent within an elastic limit. By projections 3 or 4 provided by plastic working in a part near the tip of the pleat of a plane forming the pleat apart from the cylindrical surface of the tip or a curved surface of a large radius of curvature close to a plane, the pleat is brought into contact with that next to it without being adhered maintaining a gap corresponding to the height of the projections. In this way, pleats where between the vertexes 1 of the pleats adhered to each other are provided a lot of voids in the form of an uniform long, narrow triangle 5, 6 adjacent to each other are formed.