CONTROL SYSTEM OF INTERNAL COMBUSTION ENGINE

This control device for an internal combustion engine includes: an upstream catalyst (20); a downstream catalyst (24) that is provided further downstream than the upstream catalyst in the exhaust flow direction; a downstream air-fuel ratio detection means (41) that is provided between these catalysts; a storage amount estimation means that estimates the oxygen storage amount of the downstream catalyst; and an inflow air-fuel ratio control device that controls the air-fuel ratio of the exhaust gas flowing into the upstream catalyst such that the air-fuel ratio of the exhaust gas reaches a target air-fuel ratio. In a rich control during normal operation, the target air-fuel ratio is set lean if the air-fuel ratio detected by the downstream air-fuel ratio detection means is rich, and the target air-fuel ratio is set rich if the upstream catalyst oxygen storage amount is equal to or greater than the upstream reference storage amount. If the downstream catalyst oxygen storage amount is equal to or less than a downstream lower-limit storage amount, which is less than the maximum storage amount, then the target air-fuel ratio is set lean such that the air-fuel ratio of the exhaust gas flowing out from the upstream catalyst becomes lean.