An air bypass valve failure detecting device outputs an opening command to an air bypass valve when a throttle valve suddenly closes, thereby prevent surging from occurring in a second intake passage between a turbocharger and a throttle valve. When the opening command is outputted to the air bypass valve, failure detection for the air bypass valve is permitted only if an intake air quantity of an engine immediately before a closing command is changed to the opening command is equal to or larger than a predetermined value. Thus, it is possible to prevent the failure detecting device from erroneously determining that the air bypass valve is normal although valve closing failure of the air bypass valve has occurred. This is because, when the intake air quantity is smaller than the predetermined value, surging does not occur in the second intake passage even if the valve closing failure of the airy bypass valve has occurred.
