In an internal combustion engine, an NOX selective reduction catalyst (15) is arranged in the engine exhaust passage, and a urea aqueous solution feed valve (17) is arranged in the engine exhaust passage upstream of the NOX selective reduction catalyst (15). Based on the detections results of an NOX purification rate detecting means for detecting the NOX purification rate by the NOX selective reduction catalyst (15), a urea aqueous solution feed amount detecting means for detecting the amount of feed of the urea aqueous solution, and a urea aqueous solution concentration detecting means for detecting the concentration of the urea aqueous solution, abnormalities in the NOX selective reduction catalyst (15), urea aqueous solution feed system, and urea aqueous solution are judged.