To provide a reciprocating engine for reducing a friction loss between a piston and a cylinder inner surface.

In this reciprocating engine 1, a second piston ring 27 forming an annular gas chamber 31 is downward slantingly arranged toward the thrust side 12 from the anti-thrust side 13, and the annular gas chamber 31 is gradually widely formed toward the thrust side 12 from the anti-thrust side 13, and in an upper part 33 on the thrust side 12 of an inner surface 24 of a cylinder 23, a recessed place 34 is formed to be formed longer in the circumferential direction 46 of the cylinder 23 than the direction 45 along the axis 44 of the cylinder 23, and when a piston 2 is positioned in the vicinity of the top dead center, a combustion chamber 37 above the piston 2 communicates with the annular gas chamber 31 formed in the piston 2 via a recessed space 36 of the recessed place 34.