PURPOSE: To make the best use of an advantage inherent in a claw clutch as well as to enable its off-operation to be done lightly by installing the claw clutch and a friction clutch both in a transmission system in parallel, and constituting this friction clutch to be disengaged after the off-operation of the claw clutch.

CONSTITUTION: When a claw clutch 7 and a friction clutch 8 both are in a state of being engaged, power of a first power transmission shaft 4 is parted in each of these clutches 7 and 8 parallelly and surely transmitted to a second transmission shaft 9 in utilizing an adtange inherent in the claw clutch 7. At time of off-operation, first a shift member 11 of the claw clutch 7 is operated slidingly to the right. At this time, since the power is made to flow into the friction clutch 8 as well, contact surface pressure in two projections 10a, 11a of the clutch 7 is small, so that the shift member 11 can be slidden to the right lightly, whereby the claw clutch 7 comes to a state of being disengaged. When the shift member 11 is further slidden to the right, a friction disk 14 separates from a friction disk 16, thus the friction clutch 8 comes to a state of being disengaged as well, and transmission to the second transmission shaft 9 is interrupted.