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 provided for the rack , the piston including a piston main body having a shape corresponding to the cylinder hole , and a sealing member attached to the piston main body , and the piston main body including a body , and an extended portion extending from the body toward the pinion , and wherein both end portions of the rack are coupled to the extended portions of a pair of the pistons in a manner so that the rack is deviated toward the pinion , and a space is formed between the bodies of the pair of pistons , and between a side wall of the rack and the cylinder hole , wherein the cylinder hole is partitioned by the pair of pistons 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 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 provided for the rack , the piston including a piston main body having a shape corresponding to the cylinder hole , and a sealing member attached to the piston main body , and the piston main body including a body , and an extended portion extending from the body toward the pinion , and wherein both end portions of the rack are coupled to the extended portions of a pair of the pistons in a manner so that the rack is deviated toward the pinion , and a space is formed between the bodies of the pair of pistons , and between a side wall of the rack and the cylinder hole , wherein the cylinder hole is partitioned by the pair of pistons 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 has a substantially quadrangular transverse sectional shape. a rack including a plurality of teeth configured to enmesh with the pinion ; and a piston provided for the rack , the piston including a piston main body having a shape corresponding to the cylinder hole , and a sealing member attached to the piston main body , a piston main body having a shape corresponding to the cylinder hole , and a sealing member attached to the piston main body , and the piston main body including a body , and an extended portion extending from the body toward the pinion , and a body , and an extended portion extending from the body toward the pinion , and wherein both end portions of the rack are coupled to the extended portions of a pair of the pistons in a manner so that the rack is deviated toward the pinion , and a space is formed between the bodies of the pair of pistons , and between a side wall of the rack and the cylinder hole , wherein the cylinder hole is partitioned by the pair of pistons 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 has a substantially quadrangular transverse sectional shape. <EOS>