PURPOSE: To raise the accuracy in controlling the base air-fuel ratio, by using a non-linear variable resistor which makes a small change in the value of its resistance in the vicinity of the mean value of scattering of the base air-fuel ratio as a resistor for determining the base air-fuel ratio.

CONSTITUTION: An ECU32 consisting of a micro-computer calculates a base injection period TP from the output signal of a distributor 30 and corrects the same on the basis of the output signals of a throttle sensor 16 and a water- temperature sensor 25. Further, the base injection period TP is corrected by detecting the ratio of control voltage Vaf to impressed voltage Vcc of a non- linear variable resistor 34 which determines the base air-fuel ratio and calculating a correction factor for the base injection period TP or corrected injection period from the above ratio of Vaf to Vcc. Since the non-linear variable resistor 34 is so designed that its control gradient is small at the 50% mean value of scattering of the base injection period corresponding to the base air-fuel ratio, it is enabled to easily control the value of resistance in the vicinity area of said mean value where the frequency of causing scattering of the base air-fuel ratio is high.