To provide a semiconductor device having improved performances. A semiconductor substrate has, in the surface layer portion thereof, an n+ type semiconductor region for source and an n+ type semiconductor region for drain separated from each other. The semiconductor substrate has, on the main surface thereof between the n+ type semiconductor region for source and the n+ type semiconductor region for drain, a gate electrode via an insulating film as a gate insulating film. The semiconductor substrate has, in the main surface thereof between the channel formation region below the gate electrode and the n+ type semiconductor region for drain, a LOCOS oxide film and an STI insulating. Of the LOCOS oxide film and the STI insulating film, the LOCOS oxide film is located on the side of the channel formation region and the STI insulating film is on the side of the n+ type semiconductor region DR for drain.