In an internal combustion engine, an SOx trap catalyst (11) able to trap SOx contained in exhaust gas is arranged in an engine exhaust passage upstream of an NOx storing catalyst (12). When whether the SOx trap catalyst (11) has deteriorated should be judged, the SOx trap catalyst (11) is raised in temperature in the state with the air-fuel ratio of the exhaust gas flowing into the SOx trap catalyst (11) maintained lean. At this time, the concentration of NOx released from the SOx trap catalyst (11) is detected by the NOx concentration sensor (24). When the NOx concentration is a set value or less, the catalyst is judged to have deteriorated.