To diagnose a failure of an intake-air control system without new addition of a detection signal or the like, and to enhance the reliability of a result of detection.

If a linear O2 sensor for detecting an air-fuel ratio is normal and active (S81, S83) while fuel is not cut off during an acceleration increment compensation period (S87) in which a fuel injection volume is subjected to an acceleration increment or during a deceleration decrement compensation period (S88) in which a fuel injection volume is subjected to a deceleration decrement, it is determined that a diagnosing condition is satisfied. Further, under the satisfied diagnosing condition, if an air-fuel ratio λa detected on the basis of an output value from the linear O2 sensor is smaller than a rich side determination value λR which cannot be obtained during normal operation of a dump generator valve TGV as an intake control valve (S90), or it exceeds a lean side determination value λL (S91), it is determined that a TGV system fails, and a TGV system failure flag TGVNG is set (S94).