A non-volatile semiconductor memory and three or more types of transistors are provided. A thickness of a first gate oxide film of a first transistor is larger than that of a second gate oxide film of a second transistor, and is smaller than that of a third gate oxide film of a third transistor. In a first transistor region, a first silicon oxide film is formed on a surface of a semiconductor substrate, and second and third silicon oxide films are formed on the first silicon oxide film. By removing the second and third silicon oxide films and a part of an upper layer of the first silicon oxide film, the first gate oxide film is formed from the first silicon oxide film.