To provide a reciprocating engine that does not generate attachment and accumulation of carbon in a gas chamber even if the engine repeats inducting, holding, and discharging high-temperature high-pressure combustion gas in and from the gas chamber for a long period.

A reciprocating engine 1 induces a high-temperature high-pressure combustion gas 12 at the upper part of a piston 2 into a gas chamber 4 formed by being surrounded by a top ring 5, second ring 6, second land 7 of the piston 2, and cylinder inner face 8 from a gas passage hole 23 provided on the upper section 22 of the thrust side 10 of a cylinder inner face 8 at the early stage of an expansion stroke to support the piston 2 from the thrust side 10 by the high-temperature high-pressure combustion gas 12. A half-divided ring 13 formed from a metal plate having thermal conductivity lower than the thermal conductivity of the forming material of the piston 2 is vertically movably inserted into the gas chamber 4 from the thrust side 10 under the condition of the second land 7 covered so that the half-divided ring 13 is exposed to the high-temperature high-pressure combustion gas 12 induced into the gas chamber 4.