PURPOSE: To prevent a ball from falling out while in the assembly by attaching a dog piece to either one of input and output shafts, and slidably attaching a dog clutch hub to the other shaft, and forming a stepped portion on the periphery of the dog clutch hub in the case when the ball inserted into a hole and sleeve which retains it are used to move the dog clutch hub in its axial direction.

CONSTITUTION: A dog piece 31 is fixed to either one of input and output shafts 1, 6, and a dog clutch hub 11 which is engaged with this dog piece 31 is slidably attached thereto. A ball 21 which controls the axial movement of the dog clutch hub 11 in the state where it is fitted in a hole 5 formed through the other one of the input and output shafts 1, 6, a sleeve 22 for retaining the ball, and a driving member 41 which shifts the sleeve 22 in its axial direction are used to move the dog clutch hub 11 in its axial direction; while a stepped portion 17 made of a small part 13 and a large part 16 and a space between them is formed on the periphery of the dog clutch hub 11. By this constitution, the sleeve 22 is made to abut against the stepped portion 17 of the dog clutch hub 11 so as to control its position while they are assembled; the ball 21 does not, therefore, fall outside.