A diesel oxidation catalyst, a NOx occlusion reduction catalyst, and a diesel particulate filter are connected to an exhaust pipe of an engine, downstream of an exhaust pipe injector provided on the exhaust pipe. NOx in the exhaust gas is occluded by the NOx occlusion reduction catalyst when an air-fuel ratio is lean. The occluded NOx is reduced and purified when the air-fuel ratio is rich. When a prescribed amount of particulate matter has accumulated in the diesel particulate filter, the exhaust gas temperature is raised when performing particulate matter regeneration. The exhaust gas temperature is further raised to perform a sulphur purge. The rich condition is prohibited during the particulate matter regeneration period and the sulphur purge period. When the sulphur purge is completed, the rich condition prohibition continues until the difference between the inlet temperature of the diesel oxidation catalyst and the outlet temperature of the NOx occlusion reduction catalyst becomes equal to or less than a predetermined threshold.