To set the displacement stroke of a pressure receiving body at an arbitrary value.

A piston 21 is movably provided inside the cylinder tube 20 of an air cylinder 13, and the inside of the cylinder tube 20 is partitioned by the piston 21 into first and second pressure chambers 22a, 25b. The inside of the second pressure chamber 22b is provided with a coil spring 26, a first ring- shaped plate 27, a ring body 28, a cylindrical body 29, and a second ring-shaped plate 30. When air is supplied to the first pressure chamber 22a for moving the piston 21, the coil spring 26 is elastically deformed. The second ring-shaped plate 30 is pressed by the elastic force of the coil spring 26 itself through the first ring-shaped plate 27 and the cylindrical body 29. Then, strain is produced in the second ring-shaped plate 30 in correspondence to the position of the piston 21, so that the position of the piston 21 is detected on the basis of the strain. The coil spring 26 is properly exchanged in correspondence to the piston stroke as the occasion may demand.