Disclosed is a small actuator for a robot, including an intermediate case including a driving motor and a plurality of reduction gears, a front case having a through-hole, a housing coupled to the intermediate case and including a rear case having at least one tapping hole, a rotation part that has a tapping hole, is connected to the protruding final reduction gear, and is rotated at a final reduction ratio, a first tapping screw that passes through the tapping hole of the rotation part and is fastened to a rotary shaft of the protruding reduction gear, and at least one second tapping screw that passes through one side of the front case and the intermediate case and is fastened to the tapping hole of the rear case.