PURPOSE: To secure smooth rotation of an internal combustion engine, lower the idling speed to reduce the fuel consumption, and improve idling speed control characteristic.

CONSTITUTION: In the idling state of an internal combustion engine, the target value LUsoll of rotational smoothness is set. Further, the minimum and maximum idling speed values nLmin and nLmax are set. In the case where it enters idling speed range and smoothness still exist (a), the idling speed is further reduced to nL1. When the engine temperature rises from θ11 to θ2, smoothness still exists, so that the idling speed can be reduced to nL2 (c). The smoothness control is conducted only in the case where the idling speed is minimum value and maximum value.