To significantly improve the conversion of a nitrogen oxide during dilute mixed gas combustion operation by arranging a nitrogen oxide trapping substance on the downstreams side of a dilute mixed gas combustion nitrogen oxide catalyst within an exhaust gas passage.

In this catalyst system useful for the conversion of carbon monoxide, a nitrogen oxide and a hydrocarbon present in exhaust gas and arranged in the gas passage of a dilute mixed gas combustion internal combustion engine, two components, that is, a nitrogen oxide catalyst of dilute mixed gas combination (1) and a nitrogen oxide (NOx) trapping substance (2) absorbing NOx when exhaust gas flowing into the trapping substance is dilute mixed gas and discharging absorbed NOx when oxygen concn. in inflow exhaust gas lowers are contained. In this case, the nitrogen oxide trapping substance is arranged on the downstream side of the dilute mixed gas combustion nitrogen oxide catalyst within the exhaust gas passage so that exhaust gas is exposed to the dilute mixed gas combustion nitrogen oxide catalyst before exposed to the nitrogen oxide trapping substance.