Abstract: Conventionally, an insulating film for element isolation has had a uniformly large thickness either in a memory cell area and in a peripheral circuit area so that the total film thickness of the memory cell area having a floating gate electrode, a control gate electrode, and an erase gate electrode is extremely increased, resulting in a large height difference between the memory cell area and the peripheral circuit area. The insulating film for element isolation in the peripheral circuit area should be thick, while the insulating films for element isolation in the memory cell area need not be as thick as the insulating film for element isolation in the peripheral circuit area in terms of operation.