An internal combustion engine comprises: an exhaust purification catalyst; a downstream side air-fuel ratio sensor which is arranged at a downstream side of the exhaust purification catalyst; and an air-fuel ratio control system which performs feedback control so that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst becomes a target air-fuel ratio. The air-fuel ratio control system switches the target air-fuel ratio to a lean set air-fuel ratio when the air-fuel ratio detected by the downstream side air-fuel ratio sensor becomes a rich judged air-fuel ratio or less; changes the target air-fuel ratio to a slight lean set air-fuel ratio after switching the target air-fuel ratio to the lean set air-fuel ratio and before an estimated value of the oxygen storage amount of the exhaust purification catalyst becomes a switching reference storage amount or more; and switches the target air-fuel ratio to a rich air-fuel ratio when the estimated value of the oxygen storage amount of the exhaust purification catalyst becomes the switching reference storage amount or more.