PURPOSE: To reduce fuel consumption at the time of decelerating engine and to minimize generation of noxious gases that possibly cause pollution, by controlling the amount of air supplied to engine by controlling the opening of a bypass valve to suit the revolution speed of engine.

CONSTITUTION: Analogue inputs such as output 24A of sensor 24 for detecting flow rate of suction air and output 56A of sensor 56 for detecting engine cooling water, signal 104A from switch 104 operated in association with throttle valve, and signal afforded from crank angle sensor 106 are given to control circuit 70. Input/output interface circuit 114 is afforded with input signals from ADC 102 and sensors 104, 106 and gives signals to CPU 108, which in turn gives signals to injector 12 and ignition coil 58. In the above arrangement, control circuit 70 is designed to drive bypass valve 80 in response to the difference between an aimed engine speed and actual engine speed detected by suitable means.