A three-way catalyst (20) and an NO x adsorption catalyst (22) are disposed in an engine exhaust passage. In a predetermined low-load engine operation area, combustion in a combustion chamber (5) is carried out at a lean base air-fuel ratio and an air-fuel ratio in the combustion chamber (5) is changed to a rich range at the time of discharging NO x from the NO x adsorption catalyst (22). In a predetermined high-load engine operation area, the air-fuel ratio in the combustion chamber (5) is controlled to a theoretical air-fuel ratio in a feedback manner. In a predetermined middle-load engine operation area, the combustion in the combustion chamber (5) is carried out at the base air-fuel ratio lower than the base air-fuel ratio in the low-load engine operation area and the air-fuel ratio in the combustion chamber (5) is changed to the rich range with a period shorter than a rich period of the air-fuel ratio for discharging NO x in the low-load engine operation area.