A hydrocarbon feed valve and an exhaust purification catalyst are arranged in an engine exhaust passage. A first NOX purification method which injects hydrocarbons from the hydrocarbon feed valve by a predetermined period to thereby remove NOX which is contained in the exhaust gas and a second NOX purification method which makes the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst rich to make the exhaust purification catalyst release the stored NOX when the NOX which is stored in the exhaust purification catalyst exceeds a first allowable value are selectively used. Hydrocarbons are injected from the hydrocarbon feed valve by the predetermined period, and when the NOX which is stored in the exhaust purification catalyst exceeds a second allowable value which is smaller than the first allowable value, the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst is made rich.