The NOx storage catalyst and the NOx selective reduction catalyst are arranged in the engine exhaust passage. In the engine low load operating region, a combustion is performed in the combustion chamber under a lean base air-fuel ratio and the air-fuel ratio in the combustion chamber is made rich when NOx should be released from the NOx storage catalyst. In the engine medium load operating region, the base air-fuel ratio is made smaller, a degree of richness of the air-fuel ratio is made smaller, and a period in which the air-fuel ratio is made rich is made shorter so that an amount of ammonia which is produced per unit time when the air-fuel ratio is made rich falls compared with the time of engine low load operation.