PURPOSE: To improve the wear resistance of sliding parts while reduce the cost of operating an engine by using a gas fuel as a main fuel and a coal-liquid mixture fuel as a subsidiary fuel and injecting these fuels into a combustion chamber.

CONSTITUTION: When a subsidiary fuel is fed from a fuel injection pump into a subsidiary fuel sump 28, it pushes up a subsidiary fuel needle 14 to open an injection passage 18, and is radiately injected into an injection chamber 8 from a subsidiary fuel injection port 22. After that, a main fuel needle 13 is pushed up by a fuel under pressure, opening an injection passage 20, to inject a main fuel gas in a main fuel sump 26 from an injection port 21. A slurry of coal-water or coal-oil in which about 5W10% of the total fuel is mixed, is used as the subsidiary fuel and, as for the properties of the coal, e.g., the average diameter of pulverized coal grains is set at 10μ or less, and ash content at 5% or less, and a clay at 30cst or less. Sliding parts such as a cylinder liner 3, a piston ring 4, the needles 13, 14, etc. are formed with a ceramic, etc. Thereby, the wear resistance of the sliding parts can be improved.