The internal combustion engine has an upstream side NOx selective reduction catalyst and a downstream side NOx selective reduction catalyst which are arranged in an engine exhaust passage. The upstream side NOx selective reduction catalyst has a region where the NOx removal rate becomes substantially constant when the ratio of concentration of ammonia to NOx of the inflowing exhaust rises. The internal combustion engine has an operating state in that region where the ratio of concentration of ammonia to NOx is maintained. The downstream side NOx selective reduction catalyst has an ammonia oxidation ability which is larger than the upstream side NOx selective reduction catalyst.