In an exhaust gas purifying system including two NOx catalysts disposed in series with each other on an exhaust passage, oxides occluded in the NOx catalysts can be reduced appropriately. When the oxides occluded in an upstream or downstream NOx catalyst are reduced, the surrounding atmosphere of each NOx catalyst is changed between a reducing atmosphere and an oxidative atmosphere. The air-fuel ratio of exhaust gas upstream of the upstream NOx catalyst is made lower when the surrounding atmosphere of the downstream NOx catalyst is changed into a reducing atmosphere than when the surrounding atmosphere of the upstream NOx catalyst is changed into a reducing atmosphere, and the duration of the oxidative atmosphere is made longer when the surrounding atmosphere of the downstream NOx catalyst is changed into an oxidative atmosphere than when the surrounding atmosphere of the upstream NOx catalyst is changed into an oxidative atmosphere.