A rotary actuator (1, 2) that outputs driving torque as a result of an output shaft pivoting in a rotational direction due to action of a pressure medium, the rotary actuator comprising: a case; a cylinder that is installed within the case and internally has a hollow space; an output shaft that is rotatably supported with respect to the case, has an axial direction parallel to an axial direction of the cylinder, and is installed in the hollow space; an arm (15a, 15b) that is integrated with, or fixed to, the output shaft, and extends in a radial direction of the cylinder; and a piston (14a,14b) that has a portion extending in an arc, and is installed within the cylinder and supported so as to be able to slide and be displaced with respect to the cylinder along a circumferential direction of the cylinder, wherein one end portion of the piston (14a, 14b) is connected to the arm (15a, 15b) so as to be rotatable about an axis with respect to the arm (15a, 15b), the axis being parallel with the axial direction of the output shaft and being shifted from the output shaft in the radial direction of the cylinder, the cylinder is internally provided with a first pressure chamber in which the output shaft and the arm (15a, 15b) are housed, and a second pressure chamber (26a, 26b) that is defined by the cylinder and the piston (14a, 14b) and in which another end portion of the piston (14a, 14b) that is located opposite from the end portion thereof connected to the arm (15a, 15b) is slidably installed, and as a result of a pressure medium being fed into one of the first pressure chamber and the second pressure chamber (26a, 26b) and discharged from the other, the arm (15a, 15b) is displaced in the circumferential direction of the cylinder, and the output shaft pivots in the rotational direction. a case; a cylinder that is installed within the case and internally has a hollow space; an output shaft that is rotatably supported with respect to the case, has an axial direction parallel to an axial direction of the cylinder, and is installed in the hollow space; an arm (15a, 15b) that is integrated with, or fixed to, the output shaft, and extends in a radial direction of the cylinder; and a piston (14a,14b) that has a portion extending in an arc, and is installed within the cylinder and supported so as to be able to slide and be displaced with respect to the cylinder along a circumferential direction of the cylinder, wherein one end portion of the piston (14a, 14b) is connected to the arm (15a, 15b) so as to be rotatable about an axis with respect to the arm (15a, 15b), the axis being parallel with the axial direction of the output shaft and being shifted from the output shaft in the radial direction of the cylinder, the cylinder is internally provided with a first pressure chamber in which the output shaft and the arm (15a, 15b) are housed, and a second pressure chamber (26a, 26b) that is defined by the cylinder and the piston (14a, 14b) and in which another end portion of the piston (14a, 14b) that is located opposite from the end portion thereof connected to the arm (15a, 15b) is slidably installed, and as a result of a pressure medium being fed into one of the first pressure chamber and the second pressure chamber (26a, 26b) and discharged from the other, the arm (15a, 15b) is displaced in the circumferential direction of the cylinder, and the output shaft pivots in the rotational direction.