To reduce travelling speed of a piston when the piston is moved close to a stroke end, and to prevent the piston from being pushed back just before reaching a stroke end.

On the way of stroke in which a piston 20 moves toward a rod cover 14 side as a stroke end side, a second pressure acting chamber 162 is tightly closed to increase pressure of air in the second pressure chamber 162. Since the pressure of the air in the second pressure acting chamber 162 becomes back pressure against the piston 20, the piston 20 moves toward the stroke end while decelerating. Just before the piston 20 reaches the stroke end, the second pressure acting chamber 162 is connected with a second supply/discharge conduit 32 through a second communication passage 42. Therefore, the air in the second pressure acting chamber 162 is discharged to the second supply/discharge conduit 32 through the second communication passage 42.