A control system of an internal combustion engine provided with an exhaust purification catalyst having an oxygen storage ability in an engine exhaust passage (9, 19, 21, 22, 23), the control system comprising: an upstream side air-fuel ratio sensor arranged upstream of the exhaust purification catalyst and detecting the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst; and a downstream side air-fuel ratio sensor arranged downstream of the exhaust purification catalyst and detecting the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst, wherein the control system is configured to perform lean control to intermittently or continuously make the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst a lean set air-fuel ratio leaner than a stoichiometric air-fuel ratio until an oxygen storage amount of the exhaust purification catalyst becomes a judgment reference storage amount, which is less than the maximum oxygen storage amount, and rich control to intermittently or continuously make the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst a rich set air-fuel ratio richer than the stoichiometric air-fuel ratio until an output of the downstream side air-fuel ratio sensor becomes a rich judged air-fuel ratio, which is an air-fuel ratio richer than the stoichiometric air-fuel ratio, or become less, characterized in that the control system is configured to estimate the oxygen storage amount based on an output of the upstream side air-fuel ratio sensor, the control system is configured to perform: control to switch to the rich control when the oxygen storage amount becomes the judgment reference storage amount during the time period of lean control and switch to the lean control when the output of the downstream side air-fuel ratio sensor becomes the rich judged air-fuel ratio or less during the time period of rich control, and control to set the lean set air-fuel ratio at a first intake air amount to a rich side from the lean set air-fuel ratio at a second intake air amount smaller than the first intake air amount when comparing the lean set air-fuel ratio at the first intake air amount with the lean set air-fuel ratio at the second intake air amount. an upstream side air-fuel ratio sensor arranged upstream of the exhaust purification catalyst and detecting the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst; and a downstream side air-fuel ratio sensor arranged downstream of the exhaust purification catalyst and detecting the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst, wherein the control system is configured to perform lean control to intermittently or continuously make the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst a lean set air-fuel ratio leaner than a stoichiometric air-fuel ratio until an oxygen storage amount of the exhaust purification catalyst becomes a judgment reference storage amount, which is less than the maximum oxygen storage amount, and rich control to intermittently or continuously make the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst a rich set air-fuel ratio richer than the stoichiometric air-fuel ratio until an output of the downstream side air-fuel ratio sensor becomes a rich judged air-fuel ratio, which is an air-fuel ratio richer than the stoichiometric air-fuel ratio, or become less, characterized in that control to switch to the rich control when the oxygen storage amount becomes the judgment reference storage amount during the time period of lean control and switch to the lean control when the output of the downstream side air-fuel ratio sensor becomes the rich judged air-fuel ratio or less during the time period of rich control, and control to set the lean set air-fuel ratio at a first intake air amount to a rich side from the lean set air-fuel ratio at a second intake air amount smaller than the first intake air amount when comparing the lean set air-fuel ratio at the first intake air amount with the lean set air-fuel ratio at the second intake air amount.