Automatic reduction-ratio changing apparatus

An automatic reduction-ratio changing apparatus comprises a planetary gear mechanism including a sun gear, planet gears, and an internal gear. When a load, which exceeds a predetermined torque, is applied to the internal gear, a pawl of a lock lever is engaged with a ratchet section of a housing to stop the rotation of the internal gear and lock the internal gear. When a displaceable member of an actuator is displaced from an outward route to a homeward route, the rotation of a motor is reversed, and an input shaft and a cam are rotated in a reverse direction to release from the locked state. Accordingly, the speed reduction ratio, which is transmitted from an output shaft to the actuator, is automatically switched.