PURPOSE: To assist firing and obtain the optimum combustion by a method wherein an electromagnetic wave generating device is controlled in accordance with the operating condition of the engine, such as the revolving speed of the engine, the temperature of cooling water or the like, and electromagnetic wave is projected against a target arranged at an electromagnetic wave projecting point.

CONSTITUTION: The revolution number N of the engine is detected by a disc 30, equipped with slits, and a photo-electric converting element 34 while the temperature T of the cooling water is detected by a cooling water temperature sensor 38. A control circuit 20 detects whethr these N, T are existing in the ON aera of a predetermined pattern or not and when it is in ON area, the circuit 20 puts ON the electric power source 40 of a laser oscillator 10. According to this method, the laser beams are introduced and released to project the target 14 through a semi-transparent mirror 8 and an insulating glass 4 and heat the target. Upon this time, fuel is injected into the combustion chamber and the ignition thereof may be promoted.