To securely regulate the deflection of a piston ring so as to prevent the deterioration of the piston ring by providing a projection, which can abut on the piston ring, in a piston plate, and communicating two chambers, which are formed vertically partitioning inside of a cylinder, with each other through a limiting passage formed in a piston main body.

When a piston rod 2 is moved in the compressing direction, a piston ring 13 is moved inside of a piston ring groove 11 so as to abut on a projection 18 of a piston plate 10, and a piston ring groove 11 and an upper chamber 8 are communicated with each other through a clearance 21 formed between a side surface 10a of the piston plate 10 and the piston ring 13. With this structure, since the pressurized gas sealed in a lower chamber 9 is flowed into the upper chamber 8 from both of a communicating passage 17 and a limiting passage 19 through the communicating passage 16, deflection to be generated in the piston ring 13 is regulated by the piston plate 10 so as to prevent the deterioration of the piston ring 13.