Exhaust emission control system and method

An exhaust emission control system comprises nitrogen oxide storage/reduction catalyst (7) disposed in an exhaust passage (3) of an internal combustion engine (1) and a nitrogen oxide sensor (9) positioned in the passage downstream of the catalyst. A controller performs a rich-spike operation to temporarily operate the engine at a rich air/fuel ratio each time the storage state of the catalyst satisfies a predetermined rich-spike condition during a lean-burn operation. The controller also measures the degree of degradation of the catalyst by comparing the output value of the sensor and changes the rich-spike condition so that the amount of nitrogen oxide stored in the catalyst at the time of start of the rich-spike operation during degradation determination period is made larger than that reached during a period other than the determination period.