To provide a reciprocating engine that eliminates piston ring interference in an oil ring, and generates desired lateral pressure to greatly reduce sliding friction resistance between both a piston ring and a side surface of a piston and a cylinder.

The reciprocating engine 1 includes: the piston ring 5; the 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 the side surface 8 of the piston 3 in the annular gas chamber 6 becomes greater on a thrust side than on a counter-thrust side of the piston 3; and a gas passage 15 for allowing the annular gas chamber 6 to communicate with the combustion chamber 2.