The actuator includes a housing configured to have not only a motor accommodation part, but also a gear mounting part to which a deceleration gear unit is mounted. The gear mounting part includes not only a shaft hole formed to be penetrated at a center part of a top surface of the gear mounting part, but also a ring gear part formed along in an inner circumference of the gear mounting part. The deceleration gear unit includes a worm wheel configured to have a sun gear that is rotatably supported by an outer surface of the gear mounting part and is inserted into the gear mounting part through the shaft hole, a plurality of planetary gears engaged with the sun gear and the ring gear part within the gear mounting part, and a carrier coupled to the plurality of planetary gears to be rotatable, and provided with an output shaft.