Inside an engine exhaust passage, in order from the upstream side, a hydrocarbon feed valve, oxidation catalyst, and exhaust purification catalyst are arranged. A first NOx purification method which lowers the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst by a predetermined period while maintaining it lean so as to remove NOx without causing NOx to be stored in the form of nitrates and a second NOx purification method which switches the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst from lean to rich by a period longer than the above period so as to remove NOx are selectively used in accordance with an operating state of the engine.