In a NOx purification system 1 provided with a NOx occlusion reduction catalyst 20 and a control unit 30 which executes a NOx regeneration control and a sulfur poisoning regeneration control, a binary λ sensor 26 and a reducing agent concentration sensor 25 for detecting a concentration of the reducing agent are disposed downstream of the NOx occlusion reduction catalyst 20. The level of deterioration of the NOx occlusion reduction catalyst 20 caused by sulfur poisoning is estimated on the basis of a time period Ta from a time point T3 at which a marked decrease of oxygen concentration is detected by the binary λ sensor 26 to the time point T2 at which the marked increase of the oxygen concentration is detected by the reducing agent concentration sensor 25 under the NOx regeneration control. Accordingly, the sulfur poisoning amount is accurately estimated and by performing the sulfur poisoning regeneration at an appropriate frequency, any excessive sulfur poisoning regeneration can be avoided, thereby preventing deterioration of mileage.