In a fan, a housing includes decreased thickness portions and increased thickness portions arranged alternately in a circumferential direction. A distance between a central axis and an edge increases from an air inlet toward an air outlet. An inner circumferential surface has a minimum radius smaller than a distance between the central axis and an outermost portion of the edge. At a boundary between upper and lower housing portions of the housing, the inner circumferential surface has a radius greater than the distance between the central axis and the outermost portion of the edge. The increased thickness portions include component fastening structures defining a fastening structure arranged to fit the upper and lower housing portions to each other.