In an internal combustion engine, inside of an engine exhaust passage, a hydrocarbon feed valve (16), an exhaust purification catalyst (13), an aqueous urea solution feed valve (17), and an NOx selective reduction catalyst (15) are arranged in that order. A first NOx purification method which makes the concentration of hydrocarbons flowing into the exhaust purification catalyst (13) vibrate by within predetermined ranges of amplitude and period to reduce the NOx, which is contained in exhaust gas in the exhaust purification catalyst (13) and a second NOx purification method which uses the fed aqueous urea solution to reduce the NOx in the NOx selective reduction catalyst (15) are selectively used.