In an internal combustion engine, an NO x selective reduction catalyst (15) is arranged in the engine exhaust passage and a urea aqueous solution stored in a urea aqueous solution tank (20) is fed to the NO x selective reduction catalyst (15) where the NO x is selectively reduced. Opening and closing of the urea aqueous solution refill port (40) of the urea aqueous solution tank (20) are detected and it is judged if the urea aqueous solution is refilled in the urea aqueous solution tank (20). When it is judged that the NO x purification rate has fallen to an allowable level or less at the time of engine operation right after the urea aqueous solution is refilled in the urea aqueous solution tank (20), it is judged that the refilled urea aqueous solution is abnormal.
