A control for controlling an intake air into an engine is provided. A control valve for adjusting an amount of the intake air into the engine is provided. A desired opening degree of a control valve provided in an intake air passage into the engine is determined based on a clogging coefficient. The clogging coefficient indicates a degree of clogging of the intake air passage. An opening degree of the control valve is controlled to converge to the desired opening degree. The clogging coefficient is updated based on a feedback correction amount for feedback controlling a rotational speed of the engine during idling operation. If a leakage in a blow-by gas passage that is connected between the engine and the intake air passage is detected, the update of the clogging coefficient is prohibited.