An axial fan includes an impeller and a housing including a side wall surrounding an outer circumference of the impeller. The side wall preferably includes at least one flat surface portion defined in an outer circumferential surface thereof, and a slit group including slits arranged to be spaced apart in a circumferential direction of an inner circumferential surface and extending in the same direction perpendicular or substantially perpendicular to the flat surface portion from the inner circumferential surface to the outer circumferential surface thereof. Within the slit group, a total number of slits positioned forward, with respect to a rotation direction of the impeller, of a point of contact of the inner circumferential surface with a tangent to the inner circumferential surface which is perpendicular or substantially perpendicular to a through direction of each slit is greater than a total number of slits positioned rearward of the contact point.