In a memory cell array on a main surface of a semiconductor substrate, a floating gate electrode for accumulating charges for information is arranged. The floating gate electrode is covered with a cap insulating film and a pattern of a first insulating film formed thereon. Further, over the entire main surface of the semiconductor substrate, a second insulating film is deposited so that it covers the pattern of the first insulating film and a gate electrode. The second insulating film is formed by a silicon nitride film formed by a plasma CVD method. The first insulating film is formed by a silicon nitride film formed by a low-pressure CVD method. By the provision of such a first insulating film, it is possible to suppress or prevent water or hydrogen ions from diffusing to the floating gate electrode, and therefore, the data retention characteristics of a flash memory can be improved.