An air filter includes: a pleated filter material folded into a zig-zag form at folding lines along a first direction to have peak parts and valley parts arising alternately in a second direction perpendicular to the first direction, trapping fine particles in a gaseous matter flowing in a gas flow direction perpendicular to the first direction and the second direction; a filter frame being a rectangular filter frame including one pair of first frame materials facing each other in the second direction having end parts in the first direction, and one pair of second frame materials facing each other in the first direction having end parts in the second direction, opening in the gas flow direction and housing the pleated filter material, each of the first frame materials and each of the second frame materials being combined to provide an overlap part.