To attain control of optionally changing the mixing ratio of fuel and water (an inert material) in a short time according to the magnitude of load and to suppress the production of a nitrogen oxide equivalently to conventional constitution by uniformly mixing the fuel and water (the inert material) in a combustion chamber equivalently to conventional water emulsion fuel.

The fuel is injected from a fuel injection nozzle 8 toward the water (the inert material) held in a water holding part 16. Or the fuel is injected from the fuel injection nozzle 8 toward the water (the inert material) supplied from a water supply line 15.