PURPOSE: To prevent malfunctioning of a control system due to its trouble in controlling the idling speed to a target speed by way of feedback control, by stopping feedback control when increase and decrease of intake air are caused predetermined times continuously and change of the engine speed is smaller than a predetermined level.

CONSTITUTION: An intake passage 12 of an internal-combustion engine 10 is provided with a by-pass passage 16 which by-passes a throttle valve 14, and the flow rate of intake air and hence the engine speed at the time of idling operation is controlled according to the opening of a control valve 18 interposed in the by-pass passage 16. Output signals of a vehicle speed sensor 38 and a sensor 28 for detecting the opening of a throttle valve are applied to a control circuit 26 consisting of a micro computor or the like, which controls operation of a valve control motor 20 to control the actual idling speed of engine detected by a crank angle sensor 34 to an aimed idling speed. Here, if increasing and decreasing of intake air are caused predetermined times continuously and change of engine speed is smaller than a predetermined level, it is judge that some trouble has been caused with the feedback control system and the feedback control is stopped.