To prevent the generation of noises caused by pulsation of a suction valve by adjusting a piston top clearance while keeping a moving rate of the suction valve constant.

As components of a casing 2 of a pressure vessel, the followings are provided. A front housing 3 has a rotatably attached driving shaft 8. A cylinder block 4 has a cylinder 16 housing a piston 10. A valve plate 6 has a suction valve which suctions refrigerant gas from the outside into the cylinder 16 due to elastic deformation of its own when the piston 10 is moved to a bottom dead center. A cylinder head 5 has a suction chamber 42 and a discharge chamber 43 which are partitioned from each other. A suction valve regulation means 34 which is arranged on the cylinder block 4 regulates a lifting rate of the suction valve at its open state. A piston top clearance adjusting member 9 with specified thickness is arranged between the front housing 3 and the cylinder block 4, for adjusting the top clearance of the piston 10 in the cylinder 16.