PURPOSE: To maintain the amount of the pilot fuel to a minimum value required for the stable ignition in every operating region by constantly detecting the inner pressure of the cylinder, judging the stability of the ignition timing based on this detected result, and controlling the amount of the pilot fuel for ignition according to this judgment.

CONSTITUTION: The pilot fuel such as A-oil and light oil which is press-fed by means of a fuel pump 18 is supplied from a pilot fuel tank 19 for ignition to a control valve 16 through a fuel supply tube 17. A control valve 16 is opened for the specified period of time by means of a control device 20 while a plunger 4 of a main fuel pump 3 to press-feed the methanol, i.e., the main fuel, is stopped. Thus, the pilot fuel of the specified amount is supplied to a pilot fuel passage of a fuel injection valve body 9 through the fuel supply tube 15. The stability of the ignition timing is judged based on the detected result of the in-cylinder pressure detecting device 100. Feedback control of the control device 20 is executed by the in-cylinder pressure monitoring device 101 according to the judged result.