An air cylinder fastening device has a belt and structure for belt tightening. Belt tightening structure is attached to each of the longitudinally opposite ends of the belt, and comprises first and second slider members opposed to each other in a circumferential direction of the air cylinder and a coupler member adapted to move these members close to or apart from each other. The coupler member comprises a front segment fixed to a rotor of the first slider member, a rear segment extending from the front segment toward the second slider member, a threaded part of the rear segment extending outward through a rotor of the second slider member, a control knob screwed on the threaded part and a spring interposed between the rotor and the control knob.