PURPOSE: To connect and disconnect an outer clutch positively by each forming teeth, dislocation coefficients thereof continuously change gradually and mutually in the opposite direction in the tooth width direction, to the outer clutch and a gear engaging with the clutch.

CONSTITUTION: The teeth 29a, 32a, the dislocation coefficients thereof continuously change gradually and mutually in the opposite direction in the tooth width direction, are each shaped to the outer clutch 29 and the gear (an idle gear) 32 engaging with the outer clutch 29. Tapers in the opposite direction in the tooth width direction are each formed to the outer clutch 29 and the tooth 32a of the gear 32 engaging with the clutch, and a backlash between the outer clutch 29 and the gear 32 increases when moving the clutch 29. When moving the outer clutch 29, friction with the gear 32 is decreased, movement is smoothed, and the clutch can positively be connected and disconnected.