The compressor is formed of a simple, straight cylinder clamped between end headers by means of tie bolts. Confined within the cylinder are inlet valves--one at each end thereof--and inlet ports formed in the cylinder open onto these valves. A reciprocable piston rod carries a pair of spaced-apart discharge valves thereon. The latter valves, having annular bodies, have piston ring grooves formed thereabout, to receive sealing piston rings therein; too the discharge valves serve as the gas compressing pistons.