Inside of an engine exhaust passage, a hydrocarbon feed valve (15) and an exhaust purification catalyst (13) are arranged. On the exhaust purification catalyst (13), platinum Pt (51) is carried and a basic layer (53) is formed. The concentration of hydrocarbons which flows into the exhaust purification catalyst (13) is made to vibrate within a predetermined range of amplitude and within a predetermined range of period, whereby the NOx which is contained in the exhaust gas is reduced by the exhaust purification catalyst (13). A clogging degree detecting means is provided for detecting a degree of clogging of a nozzle opening of the hydrocarbon feed valve (15). The NOx purification rate is estimated from the degree of clogging of the nozzle opening of the hydrocarbon feed valve (15) which is detected.