Reciprocating engine

A reciprocating engine (1) comprises: a piston ring (5); a piston ring (7) which defines an annular gas chamber (6) in cooperation with the piston ring (5) and which is adjacent to the piston ring (5) such that a pressure-receiving area of a side surface (8) of a piston (3) in the annular gas chamber (6) becomes greater on a thrust side (10) than on a counter-thrust side (9) of the piston (3); and gas passages (15) for allowing the annular gas chamber (6) to communicate with the combustion chamber (2).