Provided is a method of controlling an exhaust gas aftertreatment device, which can properly control timing for returning to a rich state after S purge, and timing for spraying urea water after particulate matter (PM) regeneration. A DOC (25), a NOx occlusion reduction catalyst (26), and a DPF (27) are connected to an exhaust pipe (20) of an engine (E), downstream of an exhaust pipe injector (23) provided on the exhaust pipe (20). NOx in the exhaust gas is stored by the NOx occlusion reduction catalyst (26) when an air-fuel ratio is lean. The stored NOx is reduced and purified when the air-fuel ratio is rich. When a prescribed amount of PM has accumulated in the DPF (27), the exhaust gas temperature is raised to perform PM regeneration. The exhaust gas temperature is further raised to perform S purge to purge sulfur stored in the NOx occlusion reduction catalyst (26). The rich condition is prohibited during the PM regeneration period and the S purge period. When the S purge is completed, the rich condition prohibition continues until the difference between the inlet temperature of the DOC (25) and the outlet temperature of the NOx occlusion reduction catalyst (26) becomes equal to or less than a predetermined threshold.