An exhaust purification system comprises an exhaust purification catalyst 20, an NOX sensor 46, air-fuel ratio sensor 41 downstream of the catalyst 20, and a control and diagnosis device. The device alternately sets a target air-fuel ratio to a rich air-fuel ratio and a lean air-fuel ratio and switches the target air-fuel ratio from the rich air-fuel ratio to the lean air-fuel ratio when the output air-fuel ratio of the air-fuel ratio sensor becomes a rich judged air-fuel ratio or less. The device diagnoses abnormality of the catalyst based on the output of the NOX sensor. It diagnoses abnormality of the catalyst when the air-fuel ratio of the exhaust gas flowing into the catalyst is a rich air-fuel ratio, but does not diagnose abnormality of the catalyst when the air-fuel ratio of the exhaust gas flowing into the catalyst is a lean air-fuel ratio.