In an internal combustion engine, a hydrocarbon supply valve (15) and an exhaust gas purification catalyst (13) are disposed inside the exhaust gas passage of the engine. A first NOx removal method and a second NOx removal method are selectively used, wherein the first method comprises ejecting hydrocarbons from the hydrocarbon supply valve (15) at a predetermined interval to remove the NOx contained in the exhaust gas, and in the second method, in cases when the amount of the NOx occluded in the exhaust gas purification catalyst (13) has exceeded a first allowable limit, the air-fuel ratio of the exhaust gas being introduced into the exhaust gas purification catalyst (13) is made rich, thereby discharging the occluded NOx from the exhaust gas purification catalyst (13). Alternatively, hydrocarbons are ejected from the hydrocarbon supply valve (15) at predetermined intervals, and in cases when the amount of the NOx occluded in the exhaust gas purification catalyst (13) has exceeded a second allowable limit, which is lower than the first allowable limit, the air-fuel ratio of the exhaust gas being introduced into the exhaust gas purification catalyst (13) is made rich.