A side wall of a motor casing is formed into a polygonal shape such that flat side portions and corner portions located at respective corners between the side portions are continuously joined together. A field magnet, which has an outer circumferential shape approximately coinciding with the inner circumferential shape of the motor casing, has magnet side portions and magnet corner portions, and has a rib provided on outer circumferential surfaces of the magnet side portions. The magnet is disposed, through press-fitting, within the motor casing such that at least the rib comes into contact with the inner circumferential surface of the motor casing, wherein the magnet side portions, excluding the rib, are formed such that small clearances are formed between the magnet side portions and the side portions of the motor casing.