PURPOSE: To prevent degradation of the performance of a Diesel engine, by prohibiting restriction of intake air for idling operation for a predetermined while in case that the engine is brought into idling operation just after decelerating operation of the engine is continued for more than a predetermined time.

CONSTITUTION: At steps 3W6, judgement is made whether decelerating operation of an engine is continued for more than a predetermined time. In case of YES, a flag F for prohibiting restriction of intake air for idling operation is set to 1. At a step 10, judgement is made whether the flag 1 for prohibiting restriction of intake air for idling operation is 1, and in case of YES, program proceeds to a step 11. Restriction of intake air for idling operation is prohibited for a predetermined while determined by a count value C2, and then restriction of intake air for idling operation is started. By empolying such a method, it is enabled to prevent degradation of the engine performance.