To maintain the long-time performance of a NOx catalyst by preventing the poisoning of the NOx catalyst due to SOx for the reduction of NOx in exhaust gas from an internal combustion engine, especially from a vehicular diesel engine.

Plural upstream-side passages 20a, 20b, selectively and alternately communicated with an exhaust gas passage 14 and an regenerated air passage 30 are provided at the upstream side of downstream-side passages 36a, 36b mounted with a NOx catalyst 38 to mount SOx adsorbing catalysts 22a, 22b in the upstream-side passages 20a, 20b, respectively. Exhaust gas is carried in one upstream-side passage via a switch valve 16 and SOx is removed by the SOx adsorbing catalyst and guided to the NOx catalyst at the downstream side to effectively remove NOx in the exhaust gas. On the other hand, regenerating air is carried into the other upstream-side passage 20a or 20b via the switch valve and the regenerating air or the SOx adsorbing catalyst is heated by heaters 26a, 26b to remove adsorbed sulfur oxide and recover the SOx adsorbing performance of the catalyst.