Sulfur purge control method and exhaust gas purifying system

In an exhaust gas purifying system (1) for removing NOx by a NOx occlusion reduction type catalyst (11) for exhaust gas of an internal combustion engine, the air-fuel ratio in the exhaust gas is controlled by setting the target air-fuel ratio at a predetermined first air-fuel ratio which is a rich air-fuel ratio by a sulfur purge control means (C24) after the start of sulfur purge, and thereafter, the air-fuel ratio in the exhaust gas is controlled by changing the target air-fuel ratio to the predetermined second air-fuel ratio which is a stoichiometric air-fuel ratio, when the oxygen concentration (Od) in the downstream of the NOx occlusion reduction type catalyst (11) measured by an oxygen concentration detection means (C12) becomes lower than a predetermined threshold. Thereby, sulfur component accumulated in the NOx occlusion reduction type catalyst (11) can be purged efficiently, while preventing carbon monoxide from being discharged into the atmospheric air.