PURPOSE: To suppress generation of noise in a piston sealing part and prolong the service life of the sealing part by forming a gap where the compression space side is larger than the buffer space side between a piston and cylinder and sealing the piston by said gap.

CONSTITUTION: A gap 17 whose compression space 12 side is larger than that on a buffer space 13 side is formed between a piston 5 and a cylinder 6, and sealing for the piston 5 is performed by the gap 17. Therefore, when the piston 5 and cylinder 6 are set into eccentric state, the pressure on the side where the gap 17 is reduced is made higher than that on the opposed side, and the piston 5 is pressed back, and the center of the piston 5 is allowed to coincide with the center of the cylinder 6. This action permits the piston 5 to support a piston side load, in the floated state for the cylinder 6. Therefore, the sealing where the piston 5 does not contact the cylinder 6 can be generated, and abrasion can be reduced, and also the generation of impact noise in the piston 5 part can be suppressed.