To provide a failure detecting means to relax the limit of an operation region where the failure of an AFS is detected to enable the increase of frequency of failure detection.

The failure detecting device detects the trouble of an air flow sensor for an internal combustion engine provided with the air flow sensor situated in an intake air passage; a throttle; and a pressure sensor situated downstream of the throttle, and the device is provided with a first calculating part to calculate a first intake air amount based on an output signal from the air flow sensor; and a deciding part to decide the failure of the air flow sensor through comparison between the first intake air amount and the second intake air amount. Decision of the failure is performed based on comparison between the first intake air amount calculated based on an air flow sensor output and the second intake air amount calculated based on the output of the pressure sensor situated downstream of the throttle of the intake air passage.