PURPOSE: To enhance the drivability of a car by selecting the air-fuel ratio of the mixture, at the time of accelerating and decelerating the car, to the theoretical value of air-fuel ratio and by transferrig the air-fuel ratio from this theoretical valve gradually to the air-fuel ratio which gives the optimum rate of fuel consumption.

CONSTITUTION: Air for combustion is taken in throgh a suction amount sensor 3 and a throuttle valve 4 while the fuel is fed through a fuel injection valve 6. The exhaust gas is released through an exhaust manifold 7, an exhaust pipe 8 and a catalyst converter 9 in lwhich the exhaust manifold 7 is equipped with an air-fuel ratio sensor 10 which emits a voltage at high and low levels when the air-fuel ratio sensed from the oxygen concentration in the exhaust gas is smaller and larger, respectively, than the theoretical value of air-fuel ratio. The engine 1 is equipped with a temperature sensor 11 for the cooling water. The rotational speed sensor 12 emits a pulse signal at a frequency according to the revolving speed of the crank shaft. The bypass valve 13 controls the amount of taken-in air which is out of measurement because of bypassing the suction amount sensor 3 and throttle valve 4.