A three-way catalyst (20) having an oxygen storage function and an exhaust purification catalyst (22) are arranged in the exhaust passage of an internal combustion engine. During medium-load operation of the engine the degree of lean of the air-fuel ratio in the combustion chamber (5) is increased so as to increase the oxygen storage amount of the three-way catalyst (20) to the maximum oxygen storage amount, and the air-fuel ratio in the combustion chamber (5) even after the oxygen storage amount of the three-way catalyst (20) has reached the maximum oxygen, storage amount is maintained at lean, after which the air-fuel ratio is returned to rich, and at this time, as the amount of poisoning of a noble metal catalyst when the air-fuel ratio in the combustion chamber (5) is rich increases, the amount of time for which the air-fuel ratio in the combustion chamber (5) is maintained at lean is increased.