An actuator is configured to drive a boost pressure control valve of a supercharger. The actuator includes: an electric motor; an output shaft; and a speed reducer that is configured to transmit rotation, which is outputted from the electric motor, to the output shaft after reducing a speed of the rotation outputted from the electric motor. The speed reducer has a plurality of gears that include an output gear joined to the output shaft. The plurality of gears further includes a metal gear and a resin gear of a compound gear, which are respectively formed as a large diameter gear and a small diameter gear and are integrated together in one piece. The output gear is a resin gear.