Abstract: Two or more rolling-contact bearings, each having rolling members, are aligned in an axial direction. A pinion shaft, inserted in an inner cylindrical bore of each rolling-contact bearing, is supported by a housing via the rolling-contact bearings. The pinion shaft is disposed rotatably on the output shaft via a plain bearing and is shiftable on the output shaft in the axial direction. A pinion gear is attached in a cantilever fashion to a distal end of the pinion shaft opposed to the motor. The pinion gear selectively meshes with a ring gear of an engine in a startup operation to transmit the rotational force of a motor to the ring gear.