A semiconductor device comprises a memory cell array portion and peripheral circuit portion, wherein a first insulation film including elements as main components other than nitrogen fills between the memory cell gate electrodes of the memory cell array portion, the first insulation film is formed as a liner on a sidewall of a peripheral gate electrode of the peripheral circuit portion simultaneously with the memory cell portion, and a second insulation film including nitrogen as the main component is formed on the sidewall of the peripheral gate electrode via the first insulation film, thus enabling not only the formation of the memory cell portion having high reliability, but also the formation of a peripheral circuit with good efficiency, simultaneously, and avoiding gate offset of a peripheral gate.