Filter element for filtering fluid and method of producing same

A cylindrical filter element for filtering fluid, having two-dimensional wave forms spreading radially from the axis of said cylindrical element and including a plurality of closed wave crest lines, and being produced through the steps of preparing a filter material strip shaped symmetrical with respect to a center line extending from narrower one end to the wider other end of the strip, of folding said filter strip along a plurality of folding lines substantially perpendicular to the center line to form parallel wave forms between the two ends, and of curving and closing the folded filter strip in such a manner that said narrower end is located on the inner peripheral side of the curved filter strip and said wider end on the outer peripheral side, characterized in that at least one interval between adjacent two of said folding lines is different from at least one of the other intervals, and thus, at least one crest line or bottom line of said two-dimensional wave form is different from at least one of the other crest lines or bottom lines with respect to their heights.