A filter element of the present invention provides low pressure loss, superior filter performance and bonding of filtering material, and compactness. An inlet path of a nearly semicircular configuration which is formed in the width direction of a filter element is open at its upper side and closed at its lower side. An outlet path is closed at its upper side and open at its lower side. Corrugated filtering material has continuous mountain portions and valley portions. The mountain portions have a large radius of curvature R1, and the valley portions have a small radius of curvature R2. The valley portions are bonded to a flat filtering material. The open surface area of the inlet path is larger than the closed surface area of the outlet path 16. Preferably, R1/R2 is set so as to be within the range of 1.5 to 3.0.