A base injection pulse for regulating an injecting time of an injector is determined by the number of revolutions of an engine and the quantity of an intake air. The base injection pulse is calibrated or compensated as follows;T.sub.i =T.sub.p (1+K.sub.a +K.sub.b)whereinT.sub.i : a pulse width of the calibrated injection pulse, which determines the injecting time of the injector;T.sub.p : a pulse width of the base injection pulse;K.sub.a : a calibration coefficient determined by the level of an acceleration; andK.sub.b : a calibration coefficient determined by the level of a deceleration preceding to the acceleration.