A method of manufacturing a semiconductor device comprises forming a first insulator in the first area of a substrate and a second insulator formed in a second area of the substrate; forming an etching preventing film extending over the first device region surrounded by the first area and the second device region surrounded by the second area removing the etching preventing film from the first device region and first area forming a first gate insulating film over the first device region while the second device region and the second area are covered by the etching preventing film; removing the etching preventing film over the second device region and the second area forming a second gate insulating film over the second device region; and forming a first gate electrode on the first gate insulating film and forming a second gate electrode on the second gate insulating film.