EXHAUST PURIFICATION SYSTEM OF INTERNAL COMBUSTION ENGINE

In an engine, a hydrocarbon feed valve (15) and an exhaust purification catalyst (13) are arranged in the engine exhaust passage. A first NO x purification method which injects hydrocarbons from the hydrocarbon feed valve (15) by a predetermined period to thereby remove NO x which is contained in the exhaust gas and a second NO x purification method which makes the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst (13) rich to make the exhaust purification catalyst (13) release the stored NO x when the NO x which is stored in the exhaust purification catalyst (13) exceeds a first allowable value are selectively used. Hydrocarbons are injected from the hydrocarbon feed valve (15) by the predetermined period, and when the NO x which is stored in the exhaust purification catalyst (13) 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 (13) is made rich.