To provide a means for overcoming a peeling moment at the time of start of a clutch and a rotation/translation motion converting and transmitting device placed at the rear of an electric motor used as a drive motor.

The clutch 16 is formed as a claw clutch, and comprises two circular disks 38 and 40. Both circular disks are positioned parallel with each other, axially aligned with each other, and disposed at an interval therebetween. The circular disks 38 and 40 can be rotated by a free rotation angle by claws 42 and 44 disposed on the opposed end face sides thereof along the circumferential direction of both circular disks 38 and 40, and the claws 42 and 44 form a rotation stopper for limiting the free rotation angle. The rotation stopper limits the free rotation angle of the drive disk 38 relative to an output disk 40 in both rotating directions.