A rotary actuator that outputs a driving torque as a result of an output shaft pivoting in a rotational direction due to an action of a pressure medium, 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 inside 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 the cylinder has a plurality of cylinder blocks formed in a divided state, and the cylinder is integrally assembled by the plurality of cylinder blocks being put together along the axial direction of the cylinder, inside the cylinder, 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 to which a piston head portion provided at one end of the piston (14a, 14b) is opposed are provided, the cylinder is provided with a piston chamber that houses the piston (14a, 14b) supported so as to be able to slide and be displaced with respect to the cylinder and that defines the second pressure chamber (26a, 26b), between cylinder blocks that are adjacent to each other in the axial direction of the cylinder, the piston (14a, 14b) is provided with a connecting portion that is rotatably connected to the arm (15a, 15b) via a rotary shaft at an end that is opposite to the one end, inside the case, a third pressure chamber (35a, 35b) that is in communication with the second pressure chamber (26a, 26b) is provided on at least one of both sides of the cylinder in the axial direction of the cylinder, as a result of the pressure medium being supplied to one of the first pressure chamber and the second pressure chamber (26a, 26b) and discharged from the other of the first pressure chamber and the second pressure chamber (26a, 26b), the arm (15a, 15b) is displaced in the circumferential direction of the cylinder, and the output shaft pivots in the rotational direction, and when the pressure medium is supplied to the second pressure chamber (26a, 26b), the pressure medium is also supplied to the third pressure chamber (35a, 35b), and the cylinder is biased due to the action of the pressure medium; wherein a plurality of the pistons (14a, 14b) is provided, and the plurality of pistons (14a, 14b) is arranged in line along an axial direction of the output shaft. 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 inside 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 the cylinder has a plurality of cylinder blocks formed in a divided state, and the cylinder is integrally assembled by the plurality of cylinder blocks being put together along the axial direction of the cylinder, inside the cylinder, 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 to which a piston head portion provided at one end of the piston (14a, 14b) is opposed are provided, the cylinder is provided with a piston chamber that houses the piston (14a, 14b) supported so as to be able to slide and be displaced with respect to the cylinder and that defines the second pressure chamber (26a, 26b), between cylinder blocks that are adjacent to each other in the axial direction of the cylinder, the piston (14a, 14b) is provided with a connecting portion that is rotatably connected to the arm (15a, 15b) via a rotary shaft at an end that is opposite to the one end, inside the case, a third pressure chamber (35a, 35b) that is in communication with the second pressure chamber (26a, 26b) is provided on at least one of both sides of the cylinder in the axial direction of the cylinder, as a result of the pressure medium being supplied to one of the first pressure chamber and the second pressure chamber (26a, 26b) and discharged from the other of the first pressure chamber and the second pressure chamber (26a, 26b), the arm (15a, 15b) is displaced in the circumferential direction of the cylinder, and the output shaft pivots in the rotational direction, and when the pressure medium is supplied to the second pressure chamber (26a, 26b), the pressure medium is also supplied to the third pressure chamber (35a, 35b), and the cylinder is biased due to the action of the pressure medium; wherein a plurality of the pistons (14a, 14b) is provided, and the plurality of pistons (14a, 14b) is arranged in line along an axial direction of the output shaft.