An internal combustion engine, in which an NOx selective reducing catalyst (15) is arranged inside an engine exhaust passage, and aqueous urea stored in an aqueous urea tank (20) is fed to the NOx selective reducing catalyst (15) to selectively reduce the NOx. The aqueous urea tank (20) comprises a main tank (20a) and a sub tank (20b) arranged inside the main tank (20a). The aqueous urea in the sub tank (20b) is sent to an aqueous urea feed valve (17). When the aqueous urea tank (20) should be refilled with aqueous urea, the sub tank (20b) is refilled with aqueous urea. It is detected by a level sensor (40) if the aqueous urea in the aqueous urea tank (20) is refilled. When it is judged that the NOx purification rate falls below an allowable level at the time of engine operation right after the aqueous urea in the aqueous urea tank (20) has been refilled, it is judged that the refilled aqueous urea is abnormal.