Inside of an engine exhaust passage, in order from an upstream side, a hydrocarbon feed valve oxidation catalyst exhaust purification catalyst, and NOX selective reduction catalyst are arranged. By lowering the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst by a predetermined period while maintaining it lean, the NOX is reduced in the exhaust purification catalyst and the NOX which was not reduced at the exhaust purification catalyst is reduced by the ammonia which is adsorbed at the NOX selective reduction catalyst. The air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is sometimes switched from lean to rich. At this time, the ammonia which was generated at the exhaust purification catalyst is adsorbed at the NOX selective reduction catalyst.