In an internal combustion engine, an NOx selective reduction catalyst (15) is disposed within an engine exhaust gas passage. Urea is supplied to the NOx selective reduction catalyst (15), and ammonia produced from the urea is adsorbed onto the NOx selective reduction catalyst (15). NOx contained in an exhaust gas is selectively reduced mainly by the adsorbed ammonia. An NOx sensor (29), which can detect NOx and ammonia contained in the exhaust gas, is disposed within the engine exhaust gas passage located downstream of the NOx selective reduction catalyst (15). Whether or notthe amount of ammonia adsorbed on the NOx selective reduction catalyst (15) is saturated is determined based on the value detected by the NOx sensor (29) during the stop of the supply of a fuel into the engine in reduced speed operation.