PURPOSE: To minimize the variation of the rate of fuel consumption resulting from the variation of the number of revolutions of the engine at the time of starting the engine by a method wherein the on-duty ratio of a fuel solenoid for an engine starting heater is determined by a predetermined number of revolutions of the engine.

CONSTITUTION: There are provided in the engine 1 a crank angle senser 10, a negative pressure senser 8, a water temperature senser 9 and a sucking air temperature senser 11, respectively, and the signal from each of the sensers is inputted to a control unit 12. During the ordinary operation of the engine, a main solenoid 3 and a slow air bleed solenoid 4 are controlled to have a proper air-fuel ratio by the unit 12. Further, when the engine is cold-start condition and warming-up condition as detected by the water temperature senser 9, the fuel solenoid 5 is controlled on the base of the duty ratio determined by a predetermined number of revolutions of the engine which is stored in advance.