PURPOSE: To miniaturize a device by driving a clutch claw body integrally with a first passive gear with drive of an ordinary first passive gear alone, while releasing engaging with each passive gear clutch claw of both the cam and claw bodies in speed change of the second passenger drive and rotating both these claw bodies as one body.

CONSTITUTION: At time of shift selection where a first passive gear 29 alone is driven, a clutch claw 33 of a clutch claw body 32 and a cam claw 37 of a cam claw body 36 both are engaged with a passive clutch claw 30 of the first passive gear 29. In addition, at time of shift selection when the first passive gear 29 and a second passive gear 27 are rotated and driven, the cam claw body 36 and a clutch claw body 32 are slidden to a position where the cam claw 37 and the clutch claw 33 disengages with the passive gear clutch claw 30 by means of a speed difference between these first and second passive gears 29, 27, while the cam claw body 36 integrally rotating with the second passive gear 27 and the clutch claw body 32 are coupled together freely rotating at a coupling part 41. With this constitution, power transmission selecting for a driven shaft is performed smoothly and speedily, thus structural simplification can be realized.