PURPOSE: To set variably a target rotational speed in feed back control of idling rotational speed with a simple structure so that it may match increasing/ decreasing of an outside load with good accuracy.

CONSTITUTION: A deviation (ΔQ) between an intake air flow amount (Q) detected by an air flow meter and a basic air flow amount (Qcomp) corresponding to a target rotational speed (NSET) is found out during feed back control of idling rotational speed (S13). When it is in the relation of Q>Qcomp, the target rotational speed (NSET) is increasingly-corrected, since an actual outer load is larger than a basic outer load corresponding to the basic air flow amount (Qcomp). On the contrary when it is in the relation of Q<Qcomp, the target rotational speed (NSET) is decreasingly-corrected (S14, S15). The abovementioned corrected and set target rotational speed (NSET) is compared with actual engine rotational speed (N), and the opening of an idling control valve is adjusted in a direction for making the actual rotational speed (N) to approach the target rotational speed (NSET) (S16, S17, S18).