A starter motor for starting up an engine mounted to a straddled vehicle. The starter motor includes a rotor, a brush in contact with the rotor to cause a current supplied thereto, and a movable permanent magnet arranged to oppose a core of the rotor with a gap therebetween and to be movable in a circumferential direction of the rotor within an angle range. The angle range includes a retarded angle position and an advanced angle position in which the movable permanent magnet is displaced respectively in a retarded and advanced angle direction relative to the brush. The movable permanent magnet is located in the advanced angle position when the rotor starts to rotate upon a supply of the current to the rotor, and to be moved in the retarded angle direction to the retarded angle position within a period in which the rotor is rotating with the current supplied to the rotor.