A semiconductor device manufacturing method comprises: forming a gate insulative film on a semiconductor substrate by: forming a first nitride film on the substrate; forming a first oxide film and a second oxide film, the first oxide film being between the substrate and the first nitride film, the second oxide film being on the first nitride film; and nitriding the second oxide film to form, on the first nitride film, one of either: a second nitride film or an SiON film; and forming a gate electrode on the gate insulative film; wherein the equivalent oxide thickness of the gate insulative film is equal to or less than 1 nm.