A helical spline section 61 is formed on a drive shaft 32 rotated by a motor 11 via a planetary gear mechanism 12, and an axially movable overrunning clutch 13 is mounted to the helical spline section 61. A clutch inner 57 of the overrunning clutch 13 is formed integrally with a pinion 14, and the pinion 14 engages with an idle gear 15. Between the idle gear 15 and the clutch cover 68 of the overrunning clutch 13, there is provided a synthetic resin slip plate 95. The slip plate 95 is provided with slide-contact surfaces 95a and 95b coming into contact with the idle gear 15 and the clutch cover 68. On each of the slide-contact surfaces 95a and 95b, there is provided concavely an annular grease reservoir 96 holding grease on each of the slide-contact surfaces. Thereby, the lubricating ability of a spacer provided between the slide-contact members having mutually different rotation numbers is improved, resulting in reducing the cost of materials and preventing the burn-in between the members.