Exhaust gas purifying facility with nitrogen oxides absorption-reduction catalyst

In an exhaust gas purifying apparatus 1 provided with a nitrogen oxides absorption-reduction catalyst 2 for executing rich spike when the integrated value NOx mass of the NOx computed from the operating state of the engine E exceeds the predetermined first reference value NOxSL. a NOx sensor 13 is provided downstream from the nitrogen oxides absorption-reduction catalyst 2 so that the predetermined first reference value NOxSL is corrected when the value RSNOx detected by the NOx sensor 13 immediately after the rich spike is executed exceeds the predetermined second reference value RSSL. In this arrangement, since the variations in the NOx absorbing amount caused by the deterioration of the catalyst is monitored using the value detected by the NOx sensor for introducing a rich spike at appropriate moments, the purifying performance with respect to NOx in exhaust gas can be improved outstandingly as a hole.