Inside of an engine exhaust passage, in order from an upstream side, a hydrocarbon feed valve (16), oxidation catalyst (13), exhaust purification catalyst (14), and NO x selective reduction catalyst (15) are arranged. By lowering the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst (14) by a predetermined period while maintaining it lean, the NO x is reduced in the exhaust purification catalyst (14) and the NO x which was not reduced at the exhaust purification catalyst (14) is reduced by the ammonia which is adsorbed at the NO x selective reduction catalyst (15). The air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst (14) is sometimes switched from lean to rich. At this time, the ammonia which was generated at the exhaust purification catalyst (14) is adsorbed at the NO x selective reduction catalyst (15).