To improve an engine.

A combustion chamber 16 of the engine is supplied with gasoline and auxiliary fuel containing water and methanol. The concentration of oxygen in the combustion chamber can be heightened by the supply of the auxiliary fuel, so that gasoline can be burnt well. Energy efficiency can thereby be improved, and incomplete combustion can be suppressed. The overheat of the combustion chamber 16 can also be suppressed by the supply of the auxiliary fuel to suppress the generation of knocking. Further, the concentration of steam in combustion gas can be heightened, so that the quantity of nitrogen oxides, carbon compound, and the like dissolved in the steam can be increased. Consequently, the content of toxic substances in the combustion gas can be lowered by removing moisture from the combustion gas and discharging it from the engine.