A cylinder is installed within a case, and an output shaft and an arm that is integrated thereto and extends in a radial direction are installed within the cylinder. A piston extending in an arc slides and is displaced in a circumferential direction of the cylinder within the cylinder. One end portion of the piston is rotatably connected to the arm. The cylinder is internally provided with a first pressure chamber in which the arm is housed and a second pressure chamber in which the other end portion of the arm is slidably installed. A pressure medium is fed into one of the first and second pressure chambers and discharged from the other, and the output shaft pivots in a rotational direction.