A rotary actuator comprising: a linear actuating mechanism configured to cause a pinion to perform a rotating operation; and a cylinder body in which a cylinder hole is formed, the linear actuating mechanism being displaceably arranged therein, wherein the linear actuating mechanism includes: a rack including a plurality of teeth configured to enmesh with the pinion; and a piston (48a, 48b) provided for the rack, the piston (48a, 48b) including a piston main body (50a, 50b) having a shape corresponding to the cylinder hole, and a sealing member attached to the piston main body (50a, 50b), and the piston main body (50a, 50b) including a body (52a, 52b), and an extended portion (54a, 54b) extending from the body (52a, 52b) toward the pinion, and wherein both end portions of the rack are coupled to the extended portions (54a, 54b) of a pair of the pistons (48a, 48b) in a manner so that the rack is deviated toward the pinion, and a space is formed between the bodies (52a, 52b) of the pair of pistons (48a, 48b), and between a side wall of the rack and the cylinder hole, wherein the cylinder hole is partitioned by the pair of pistons (48a, 48b) into the space, a first cylinder chamber, and a second cylinder chamber, wherein the cylinder body has a substantially quadrangular outer shape; and the cylinder hole (28a, 28b) has a substantially quadrangular transverse sectional shape. a linear actuating mechanism configured to cause a pinion to perform a rotating operation; and a cylinder body in which a cylinder hole is formed, the linear actuating mechanism being displaceably arranged therein, wherein the linear actuating mechanism includes: a rack including a plurality of teeth configured to enmesh with the pinion; and a piston (48a, 48b) provided for the rack, the piston (48a, 48b) including a piston main body (50a, 50b) having a shape corresponding to the cylinder hole, and a sealing member attached to the piston main body (50a, 50b), and the piston main body (50a, 50b) including a body (52a, 52b), and an extended portion (54a, 54b) extending from the body (52a, 52b) toward the pinion, and wherein both end portions of the rack are coupled to the extended portions (54a, 54b) of a pair of the pistons (48a, 48b) in a manner so that the rack is deviated toward the pinion, and a space is formed between the bodies (52a, 52b) of the pair of pistons (48a, 48b), and between a side wall of the rack and the cylinder hole, wherein the cylinder hole is partitioned by the pair of pistons (48a, 48b) into the space, a first cylinder chamber, and a second cylinder chamber, wherein the cylinder body has a substantially quadrangular outer shape; and the cylinder hole (28a, 28b) has a substantially quadrangular transverse sectional shape. a rack including a plurality of teeth configured to enmesh with the pinion; and a piston (48a, 48b) provided for the rack, the piston (48a, 48b) including a piston main body (50a, 50b) having a shape corresponding to the cylinder hole, and a sealing member attached to the piston main body (50a, 50b), a piston main body (50a, 50b) having a shape corresponding to the cylinder hole, and a sealing member attached to the piston main body (50a, 50b), and the piston main body (50a, 50b) including a body (52a, 52b), and an extended portion (54a, 54b) extending from the body (52a, 52b) toward the pinion, and a body (52a, 52b), and an extended portion (54a, 54b) extending from the body (52a, 52b) toward the pinion, and wherein both end portions of the rack are coupled to the extended portions (54a, 54b) of a pair of the pistons (48a, 48b) in a manner so that the rack is deviated toward the pinion, and a space is formed between the bodies (52a, 52b) of the pair of pistons (48a, 48b), and between a side wall of the rack and the cylinder hole, wherein the cylinder hole is partitioned by the pair of pistons (48a, 48b) into the space, a first cylinder chamber, and a second cylinder chamber, wherein the cylinder body has a substantially quadrangular outer shape; and the cylinder hole (28a, 28b) has a substantially quadrangular transverse sectional shape.