PURPOSE: To prevent generation of engine stall by prohibiting feedback control if a specific driving condition is satisfied when the idling revolution number is controlled by feeding back the flow-rate of the intake air which detours around a throttle valve.

CONSTITUTION: An idle revolution-speed control valve 26 is interposed into a by-pass passage 24 which is installed so as to detours around a throttle valve 18 in an intake passage, and is controlled, in the idle driving stage by a digital control circuit 54 according to the difference between the engine revolution number which is obtained from the output of a crank angle sensor 44 and the aimed idle revolution number. When it is detected in the control circuit 54 that the idle revolution feedback condition is satisfied, 54, the valve 26 is controlled so that the idle revolution feedback control is prohibited if the feedback condition is satisfied in the preceding time and the engine revolution number is higher by a prescribed value than the aimed idle revolution number.