Abstract: A planetary gear actuator including a sun gear, a rotary internal gear being concentric with the sun gear, a fixed internal gear being concentric with the sun gear, compound planetary gears, and a carrier connected to each of the compound planetary gears so as to be rotatable relative to each of compound planetary gears may be provided, Each of the compound planetary gears may include a first planet gear engaged with the sun gear, a second planet gear at one side of the first planet gear, engaged with the rotary internal gear, and having a smaller number of teeth than the first planet gear, and a third planet gear at another side of the first planet gear, engaged with the fixed internal gear, and having a smaller number of teeth than the first planet gear. One compound planetary gear may overlap another compound planetary gear in a thickness direction.