PURPOSE: To prevent an engine from causing undershoot and/or overshoot action of its idle speed, by setting a probable amount of idle air corresponding to the speed of the engine, in the case of the internal-combustion eigine.

CONSTITUTION: A passage M5 bypassing a throttle valve M4 provides an idle-air amount adjusting means M6 which controls an amount of idle air. An arithmetic control means M7, on the basis of detection values by a load detecting means M3 and a speed detecting means M2, performs both feedback control of speed of an engine and probable amount control, which controls the idle-air amount adjusting means M6 by the probable amount of air corresponding to a change of target speed when a load of the engine changes. In this way, the engine can be prevented from causing undershoot and/or overshoot action in its idle speed.