In cases where at the upstream side of an NOx selective reduction catalyst there is provided another catalyst, sulfur poisoning of both the catalysts is recovered in an appropriate manner. To this end, provision is made for an NH3 generation catalyst arranged in an exhaust passage of an internal combustion engine for generating NH3, the NOx selective reduction catalyst arranged in the exhaust passage at a location downstream of the NH3 generation catalyst for reducing NOx in a selective manner, an upstream side recovery unit to recover sulfur poisoning of the NH3 generation catalyst, and a downstream side recovery unit to recover sulfur poisoning of the NOx selective reduction catalyst after the sulfur poisoning of the NH3 generation catalyst has been recovered by the upstream side recovery unit.