PURPOSE: To prevent the shift of a pinion from being impeded by dust by inserting a pinion shaft into the hole on an armature shaft and supporting said shaft and shifting the pinion and the pinion shaft integrally.

CONSTITUTION: A bearing 4 is inserted under pressure into the hole on an armature shaft 1, and one end of a pinion shaft 5 is supported turnably in the axial direction. On the other end of the pinion shaft 5, a pinion 7 is fixed which engages with an engine ring gear 6, and nearly at the middle part in the axial direction, a helical spline 8 is arraneged. Through the revolution of the armature shaft 1, a driving force is transmitted to a clutch outer 9, roller 10, and a clutch inner 11. The pinion shaft 5 and the pinion 7 are revolved through the helical spline 8, and the pinion 7 shaifts and engages with the engine ring gear 6.