In an internal combustion engine, a pair of NOx storing catalysts are arranged in series in an engine exhaust passage. When causing the upstream NOx storing catalyst to release NOx and store the released NOx in the downstream NOx storing catalyst, the oxygen concentration in the exhaust gas is temporarily reduced in a state where the exhaust gas is maintained at a lean air-fuel ratio. As opposed to this, when releasing NOx from the upstream NOx storing catalyst and the downstream NOx storing catalyst and reducing the NOx, the air-fuel ratio of the exhaust gas is temporarily switched from lean to rich.