In a compression ignition internal combustion engine provided with an NOx storage-reduction catalyst on an exhaust system, when the air fuel ratio of the exhaust gas being in a lean state is controlled to a rich state, a response time (ResS) from a time point an air fuel ratio sensor detects a first air fuel ratio (AF1), which is leaner than a stoichiometric air fuel ratio (AFS), to a time point the air fuel ratio sensor detects a second air fuel ratio (AF2), which is equal to or leaner than the stoichiometric air fuel ratio (AFS) and richer than the first air fuel ratio (AF1). When the response time (ResS) exceeds a stoichiometric air fuel ratio shift reference time (StdS), a determination is made that the air fuel ratio sensor is degraded.