A cylinder is installed within a case. An output shaft is installed in a hollow space within the cylinder. A pair of pistons extend in an arc, and is supported so as to be able to slide and be displaced along a circumferential direction of the cylinder within the cylinder. The pair of pistons is able to urge an arm integrated with the output shaft from two sides of the arm. Pressure chambers into which a pressure medium is introduced are defined between the cylinder and each piston in the pair of pistons. The pressure medium is fed to one of the pressure chambers and drained from the other, and thus the arm is displaced, and the output shaft pivots in a rotational direction.