PURPOSE: To simplify mechanism, and to enhance assembling workability, by providing the intermediate gear wheel of a transmission mechanism with a circumferential groove part, and abutting the claws of a changeover arm on the above groove part, so that the gear wheel can be directly shifted in the direction parallel to the turning shaft.

CONSTITUTION: In a clutch gear 7, gear parts 7a, 7b, a projected part 7c, and a groove part 7d are provided, and a change arm 9 is rotatably provided to a base 1, and further the claws 9a of the change arm 9 are engaged with the groove part 7d, so that the clutch gear 7 can be lifted up in the direction of A when the change arm 9 is turned in the clockwise direction, thereby the projected part 7c can be enabled to be engaged with the recessed part 3c of a belt pulley 3. Thus, it becomes possible to take two positions, and to transmit tow kinds of torque, and accordingly the mechanism can be simplified, and the assembling workability can be enhanced.