One embodiment of the present invention provides an actuator comprising: a housing having a mounting space part formed therein; a motor part provided inside the housing and generating power; a reduction gear part of which a plurality of gears respectively having an internal gear and an external gear sequentially engage with each other so as to transmit the power generated in the motor part; and a rod part engaging with the gear disposed at the last position, and linearly moving according to the rotation of the gear, wherein one of the plurality of gears is formed such that the internal gear and the external gear can rotate together or rotate independently.