PURPOSE: To facilitate maintenance control of a compressor by installing a detachable cover onto the head part of a high-pressure cylinder and permitting replacement of a piston ring to be performed by demounting the cover and taking- out the piston-ring installation part outside.

CONSTITUTION: When a piston ring is replaced, a piston ring P3 and a ring holder 4 are transferred in the vicinity of the top dead point, and a cover 6 and a washer 8 are removed by unloosing each screw part, and then a bolt 5 is taken away, and the ring holder 4 and the piston P3 are separated by loosening the screw part 35 at the top part of the ring holder 4, and then a bolt is screwed into the screw part 43 formed on the ring holder, and then the ring holder is taken-out. Thus, piston rings 41 and 42 etc. can be replaced easily.