In an internal combustion engine, inside of an engine exhaust passage, a hydrocarbon feed valve (15) and an exhaust purification catalyst (13) are arranged. The concentration of hydrocarbons which flows into the exhaust purification catalyst (13) is made to vibrate by within a predetermined range of amplitude of a 200 ppm or more and within a predetermined range of period of 5 second or less, whereby the NOx which is contained in exhaust gas is reduced at the exhaust purification catalyst (13). At this time, the nitrogen-containing intermediate which is produced in the NOx reduction process is exhausted from the exhaust purification catalyst (13). An intermediate purification catalyst (14) for removal of the exhausted nitrogen-containing intermediate is arranged downstream of the exhaust purification catalyst (13).