In an internal combustion engine, an NOx selective reduction catalyst (14) is arranged in an engine exhaust passage, and an NOx storage catalyst (12) is arranged in the engine exhaust passage upstream of the NOx selective reduction catalyst (14). When the amount of NOx stored in the NOx storage catalyst (12) exceeds a predetermined allowable value, the NOx storage catalyst (12) is raised in temperature to make the NOx storage catalyst (12) release the NOx. The amount of urea feed is decreased by exactly the amount of reduction of the calculated stored NOx amount with respect to the amount of urea feed determined from the engine operating state, and the amount of urea feed is increased by exactly the amount of reduction of the calculated released NOx amount with respect to the amount of urea feed determined from the engine operating state.