PURPOSE: To perform accurate and optimum control even during the transient time of an engine, e.g. acceleration, by a method wherein an amount of air actually sucked to an engine is computed and determined based on an output signal from a suction air amount detecting means and a preceding air amount and that of m-times ago.

CONSTITUTION: Based on output signals from a hot wire type intake air flow rate sensor (AFS)2, a crank angle sensor 10, a starting switch 11, and a cooling water sensor 12, a fuel injection amount is decided by means of an ECU9, and in synchronism with a signal from a crank angle sensor 10, the pulse width of the fuel injection pulse of an injector 8 is controlled. In this case, an air mount detected by the AFS2 is sampled at the rotation frequency of an engine to determine an average air amount An. A true intake air amount En is computed by means of the average air amount An, a preceding intake air amount En-1 of the true intake air amount En of the engine, an intake air amount En-m of m-times ago of the true intake air amount En, and the preceding value of the intake air amount En-m, and the computing result serves to control a fuel injection amount.