PURPOSE: To posteriorly and supplementarily increase the initially set idling rotating speed by a simple method by calculating an additional air quantity according to the difference between the actual rotating speed of an engine and the target rotating speed and the minimum rotating speed.

CONSTITUTION: In the step S1, according to the engine rotating speed N and the throttle valve opening, the basic air value BLW is read out. In the step S2, according to a coolant temperature TKW or the like, an additional air value ZTLW is read out. In the step S3, according to the sum of the basic air value and the load air value, the temperature correction and the load corrected air value TLW are calculated. In the step S4, according to the coolant temperature, the idling target rotating speed NSOLL is calculated. In the step S5, the idling target rotating speed is subtracted from the minimum rotating speed NMIN to calculate the rotating speed difference NDIFF. In the step S6, according to the rotating speed difference, an additional air quantity ALW is calculated. In the step S7, the additional air value is added to the air value to calculate the final air value GLW.