PURPOSE: To facilitate fine control and to improve stability of idling by a method wherein during a fluctuation in the number of idle revolutions, the one or both of pulse signals for fuel from a control unit to an injector and air are increased or decreased.

CONSTITUTION: A control unit 45 inputs the number of revolutions and an accel opening from sensors 42 and 43 by means of an operation condition detecting part 46. A fuel injection amount and an injection amount of pressurized air, by means of which a given amount of fuel is extruded are calculated by a computing part 47 and a calculating part 48, respectively. Corresponding pulse signals Tf and Ta are outputted from a fuel pulse output part 49 and an air pulse output part 50 to drive an injector 10. In which case, when idle operation is detected by an idle detecting part 51, the two calculating parts 47 and 48 output corresponding fuel pulse and air pulse. When a fluctuation in the number of idle revolutions is decided by a number of idle revolutions deciding part 52, a correction signal by means of which the fuel pulse Tf is increased and decreased is outputted to a fuel pulse output part 49. This constitution enables improvement of stability of idling.