PURPOSE: To introduce fuel vapor and water vapor simultaneously by arranging an adjustable inlet valve for fuel opening at a top dead point of a piston in a combustion chamber.

CONSTITUTION: An intake valve 6 and an exhaust valve 7 are arranged and an inlet 8 is mounted in a cylinder lid 5 in a cylinder head 1. A mixing conduit 9 having an adjustable inlet valve 10 is connected to the inlet 8. A mixture of fuel vapor/water vapor generated by a vaporizing device 11 is supplied to a cylinder chamber 14 via the mixing conduit 9 and the inlet valve 10 near a top dead point of a piston 3 at the end of the combustion stroke. Then the inlet valve 10 is closed, the fuel vapor/water vapor mixture mixed with combustion air is ignited by an ignition device 17. In this way, as fuel vapor can be introduced simultaneously with water vapor, the combustion temperature in the cylinder chamber 14 is lowered due to the existence of water vapor when fuel vapor is combusted.