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 configured to detect an air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst, a downstream side air-fuel ratio sensor arranged downstream of the exhaust purification catalyst and configured to detect an air-fuel ratio of exhaust gas flowing out from the exhaust purification catalyst, and an oxygen storage amount acquiring means for acquiring a storage amount of oxygen stored in the exhaust purification catalyst, wherein the control system is configured to perform normal operation control including lean control for continuously or discontinuously making the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst a lean set air-fuel ratio leaner than the stoichiometric air-fuel ratio until an oxygen storage amount (OSAsc) of the exhaust purification catalyst becomes a judgment reference storage amount (Cref), which is a maximum oxygen storage amount (Cmax) or less, or becomes more, and rich control for continuously or discontinuously making 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 becomes less, the normal operation control includes control switching to the rich control during the time period of the lean control when the oxygen storage amount (OSAsc) becomes the judgment reference storage amount (Cref) or more and switching to the lean control during the time period of the rich control when the output of the downstream side air-fuel ratio sensor becomes the rich judged air-fuel ratio or less, wherein the oxygen storage amount constantly fluctuates up and down, so the oxygen storage ability of the exhaust purification catalyst is kept from falling, the normal operation control includes judgment reference decreasing control decreasing the judgment reference storage amount (Cref) in the lean control when during the time period of performing the lean control, the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst becomes a lean judged air-fuel ratio, which is an air-fuel ratio leaner than the stoichiometric air-fuel ratio, or more, and the control system is configured to judge that the exhaust purification catalyst is abnormal when the judgment reference storage amount (Cref) becomes less than a predetermined deterioration judgment value (CCref). an upstream side air-fuel ratio sensor arranged upstream of the exhaust purification catalyst and configured to detect an air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst, a downstream side air-fuel ratio sensor arranged downstream of the exhaust purification catalyst and configured to detect an air-fuel ratio of exhaust gas flowing out from the exhaust purification catalyst, and an oxygen storage amount acquiring means for acquiring a storage amount of oxygen stored in the exhaust purification catalyst, wherein the control system is configured to perform normal operation control including lean control for continuously or discontinuously making the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst a lean set air-fuel ratio leaner than the stoichiometric air-fuel ratio until an oxygen storage amount (OSAsc) of the exhaust purification catalyst becomes a judgment reference storage amount (Cref), which is a maximum oxygen storage amount (Cmax) or less, or becomes more, and rich control for continuously or discontinuously making 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 becomes less, the normal operation control includes control switching to the rich control during the time period of the lean control when the oxygen storage amount (OSAsc) becomes the judgment reference storage amount (Cref) or more and switching to the lean control during the time period of the rich control when the output of the downstream side air-fuel ratio sensor becomes the rich judged air-fuel ratio or less, wherein the oxygen storage amount constantly fluctuates up and down, so the oxygen storage ability of the exhaust purification catalyst is kept from falling, the normal operation control includes judgment reference decreasing control decreasing the judgment reference storage amount (Cref) in the lean control when during the time period of performing the lean control, the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst becomes a lean judged air-fuel ratio, which is an air-fuel ratio leaner than the stoichiometric air-fuel ratio, or more, and the control system is configured to judge that the exhaust purification catalyst is abnormal when the judgment reference storage amount (Cref) becomes less than a predetermined deterioration judgment value (CCref).