An exhaust purification system of an internal combustion engine comprises an upstream side catalyst (20), a downstream side catalyst (24), a downstream side air-fuel ratio sensor (41) provided between the upstream side catalyst (20) and the downstream side catalyst (24), and a control device (31) 31 able to control an air-fuel ratio of exhaust gas flowing into the upstream side catalyst (20) as air-fuel ratio control. In the air-fuel ratio control, the control device (31) switches the air-fuel ratio of the exhaust gas to the lean air-fuel ratio when the output air-fuel ratio of the downstream side air-fuel ratio sensor (41) becomes the rich judged air-fuel ratio or less and switches the air-fuel ratio of the exhaust gas to the rich air-fuel ratio when the oxygen storage amount of the upstream side catalyst (20) becomes the switching reference storage amount or more. During the air-fuel ratio control, the control device (31) increases the concentration of NO x in the exhaust gas flowing into the upstream side catalyst (20) when the oxygen storage amount of the downstream side catalyst (24) becomes the limit storage amount or less as control for increasing NO x . As a result, NO x is kept from flowing out from the downstream side catalyst (24).