To provide an actuator enabling to lessen the capacity of the full output of an electric motor and to reduce the weight and dimensions of the electric motor, compared with a conventional one.

In the actuator comprising a hydraulic cylinder 110 having a piston 111 and a cylinder body 112, an oil pump 120 for supplying the hydraulic cylinder 110 with working oil, the electric motor 130 having the oil pump 120 for pumping out the oil in response to a revolutional speed, a position sensor 140 for detecting the position of the piston 111 with respect to the cylinder 112, and a controller 150 for controlling the revolutional speed of the electric motor 130, a cylinder chamber 110b, wherein the piston 111 is positioned in the direction in which a load in the main flying direction of a aircraft is applied from a rudder face 501, is made larger than another cylinder chamber 110a in a pressure receiving area of the piston 111.