PURPOSE: To control a mixture of fuel steam and water vapor in high precision by injecting it to preliminarily compressed combustion air under lower pressure than the maximum compression pressure and adjusting timing to start injection in accordance with viscosity of the mixture and a pressure difference of mixture pressure and compression pressure of a cylinder chamber.

CONSTITUTION: A steam mixture generator 1 to produce a mixture of fuel steam and water vapor is devised so that a water conduit 2 and a fuel conduit 3 on which a water pump 5, a fuel pump 4 and solenoid valves 6, 7 for measurement are respectively interposed are connected to each other and water and fuel introduced through these conduits 2, 3 are vaporized by heat of exhaust gas 8. The mixture of water vapor and fuel steam is introduced to a solenoid injection valve 12 through a conduit 11, and it is blown out to previously compressed combustion air in a cylinder chamber 13 in a compression process of an engine 14. At this time, the mixture is injected under lower pressure than the maximum compression pressure, and starting of injection is adjusted in accordance with desired viscosity of the mixture and a pressure difference between mixture pressure and compression pressure of the cylinder chamber 13.