An improvement is achieved in the performance of a semiconductor device. Over a first insulating film formed over a main surface of a semiconductor substrate located in a memory formation region and having an internal charge storage portion and over a second insulating film formed over the main surface of the semiconductor substrate located in a main circuit formation region, a conductive film is formed. Then, in the memory formation region, the conductive film and the first insulating film are patterned to form a first gate electrode and a first gate insulating film while, in the main circuit formation region, the conductive film and the second insulating film are left. Then, in the main circuit formation region, the conductive film and the second insulating film are patterned to form a second gate electrode and a second gate insulating film.