a motor with a rotating shaft;

a first gear which rotates in response to input of torque from the rotating shaft of the motor;

an eccentric shaft which connects with the first gear and includes a support which is offset from a rotating shaft of the first gear in a radial direction thereof;

a transmission gear which is retained by the support and includes a stopper protrusion protruding toward the first gear, the transmission gear revolving around the rotating shaft of the first gear following rotation of the first gear along with the eccentric shaft;

an output member which rotates in response to revolution of the transmission gear;

a rotation stopper on which the stopper protrusion slides in contact therewith to stop the transmission gear from rotating around an axis thereof; and

an interference avoider which is disposed between the rotation stopper and the transmission gear and works to avoid physical interference between a corner of the rotation stopper which faces the stopper protrusion and a base end portion of the stopper protrusion, wherein

the interference avoider is arranged between the rotation stopper and the transmission gear, and

the interference avoider is made of a member discrete from the rotation stopper and the transmission gear and has an annular or spherical shape.