PURPOSE: To prevent the stopping of during its low-speed operation, by holding a bypass valve at least at an opening being maintained before starting deceleration in case that difference between actual engine speed and an aimed engine speed is greater than a specified range.

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 signal to CPU 108, which in turn gives signals to injector 12 and ignition coil 58. In the above arrangement, control circuit 70 is designed such that bypass valve 80 is held at an opening being maintained before starting deceleration of engine in case that difference betwen actual engine speed and an aimed engine speed is greater than a specified ragne.