In an internal combustion engine, an upstream side air-fuel ratio sensor (23), hydrocarbon feed valve (15), exhaust purification catalyst (13), and the downstream side air-fuel ratio sensor (24) are arranged in an engine exhaust passage in that order from the upstream side. If hydrocarbons are fed from the hydrocarbon feed valve (15), the air-fuel ratio which is detected by the downstream side air-fuel ratio sensor (24) changes to the rich side from the reference air-fuel ratio which is detected when hydrocarbons are not fed from the hydrocarbon feed valve (15). The amount of hydrocarbons which are fed from the hydrocarbon feed valve (15) and which slip through the exhaust purification catalyst (13) is detected from the air-fuel ratio difference between the air-fuel ratio detected by the upstream side air-fuel ratio sensor (23) and the reference air-fuel ratio detected by the downstream side air-fuel ratio sensor (24).