A semiconductor device includes a first, second, and third MIS transistors of a first conductivity type respectively including a first, second, and third gate electrodes on a first, second, and third active regions of a semiconductor substrate with a first, second, and third gate insulating films interposed therebetween. The first gate insulating film is formed of a first silicon oxide film and a first high-k insulating film on the first silicon oxide film. The second gate insulating film is formed of a second silicon oxide film and a second high-k insulating film on the second silicon oxide film. The third gate insulating film is formed of a third silicon oxide film and a third high-k insulating film on the third silicon oxide film. The second silicon oxide film has a same thickness as the first silicon oxide film, and a greater thickness than the third silicon oxide film.