Abstract: An actuator comprises a housing member including a motor compartment and a gear compartment, the gear compartment having inner teeth formed on the inner periphery thereof, a motor having a rotary shaft with a motor gear mounted thereon, the motor being housed in the motor compartment in such a way as to permit the motor gear to project into the gear compartment, an output shaft projecting outwardly from the housing member, a larger diameter gear housed in the gear compartment which is rotatable about the output shaft and held in mesh engagement with the motor gear, a smaller diameter gear disposed coaxially with the larger diameter gear and rotatable in conjunction with the rotation of the larger diameter gear, a planet gear held in mesh engagement with both the smaller diameter gear and the inner teeth formed in the gear compartment so that the planet gear can shift its engagement position with respect to the inner teeth as the smaller diameter gear rotates, and a rotary member engaging the planet gear for r