In an axial fan, a housing includes a side wall arranged to surround an outer circumference of an impeller, and a substantially square or substantially rectangular flange arranged to project radially outward from an outer circumferential surface of the side wall. The side wall preferably includes three slit groups each including a plurality of slits arranged in a circumferential direction and arranged to extend through the side wall from an inner circumferential surface to the outer circumferential surface thereof. Two of the slit groups are defined in portions of the side wall which correspond to two adjacent corner portions in an upper half portion of the flange, while the remaining slit group is defined in a portion of the side wall which corresponds to a lower half portion of the flange. The upper and lower half portions are divided at a line parallel or substantially parallel to two opposing sides of the flange and passing through a central axis.