PURPOSE: To correctly diagnose abnormal condition such as the opening shortage of an idling control valve by opening/closing the idling control valve in a diagnostic range forcibly, and providing the abnormal condition judging means of the idling control valve in the case where the change degree of an intake air amount generated at this time is smaller than a regulated value.

CONSTITUTION: In an engine, an auxiliary intake air amount bypassing an intake throttle valve 31 is regulated by an idling control valve 32. On the other hand, rotational speed of an engine and an idling time are detected by respective detecting means 33, 34. The idling control valve 32 is feed back-controlled by a means 35 so as to make an idling rotational speed nearly a target rotational speed. In such a constitution, a prescribed diagnostic range is judged by a means 36. The opening degree of the idling control valve in the diagnostic range is changed to a prescribed mode by a forcibly opening/closing means 37. Change of an intake air amount following with opening change of the idling control valve 32 is detected by a detecting means 38. As a result, abnormal condition of the idling control valve 32 is judged by a abnormal condition judging means 39 when the change degree of the intake air amount is smaller than a regular value.