The present invention provides a scheme for determining an abnormality of an intake air system with higher accuracy in an engine an intake air amount is controlled by adjusting a lift amount of an intake valve. A throttle opening area is determined based on a current intake air amount. A feedback correction is performed on the throttle opening area. The feedback correction includes determining a correction amount of the throttle opening area such that the gauge pressure inside the intake manifold converges to a desired gauge pressure and then adding the correction amount to the throttle opening area. A feedback control is performed on an air-fuel ratio. The feedback control includes determining a correction amount of the air-fuel ratio based on a detection value of an air-fuel ratio detecting means provided in an exhaust manifold. An abnormality of the intake air system is determined during an idling operation based on the correction amount of the throttle opening area and the correction amount of the air-fuel ratio.