A filter element for use in an oil filter is composed of main fibers of at least 5 .mu.m diameter and micro fibers of less than 5 .mu.m diameter. The micro fibers are filled in a density of 0.005 to 0.02 g/cm.sup.3. A slurry including the fibers is drawn into a suction unit through suction openings to form a molded body. The filter element is given a density gradient such that the micro fibers are denser in the inner periphery than in the outer periphery. Following the drawing step, the molded body together with the inner mold is removed for subsequent dehydration and heating. During those processes the inner mold is kept located within the molded body.