To provide a reciprocating internal combustion engine which largely reduces the generated amount of nitrogen oxide and secures a stable engine operation state while restraining corrosion or deterioration of fuel economy with a simple structure.

A diesel engine 11 is provided with: a combustion chamber 15; a fuel injection nozzle 23 for injecting fuel into the combustion chamber 15; an intake passage 17 for supplying air to the combustion chamber 15; a vapor injection nozzle 24 for supplying vapor into the combustion chamber 15; and a vapor supply control device 33 for controlling an injection state of the vapor injection nozzle 24. The vapor supply control device 33 supplies the vapor into the combustion chamber 15 in correspondence with an operation state of the diesel engine 11. The vapor is changed to a supercritical state in the combustion chamber 15, and then, the fuel is burned with a supercritical mixed fluid of the air and the water, and the combustion gas is used as a working fluid of the diesel engine 11.