PURPOSE: To reduce the blow-by gas quantity and sharply reduce the lubricant consumption by using tapered pressure rings for both a top ring and a second ring in a piston set structure inserted into the high-pressure side cylinder of a two-stage reciprocating compressor.

CONSTITUTION: A piston set structure inserted into the low-pressure side cylinder of a lubrication type two-stage reciprocating compressor is made different from a piston set structure inserted into the high-pressure side cylinder. Tapered pressure rings are used for both a top ring 3 and a second ring 4 of the piston ring structure inserted into the high-pressure side piston, and an oil scraping ring is used for a side ring. The abutments of the pressure rings 3, 4 are formed into a step cut shape. A tapered pressure ring is used for a top ring of the piston set inserted into the low-pressure side cylinder, a tapered and undercut pressure ring is used for a second ring, and an oil scraping ring is used for a side ring.