An ultraviolet-erasable nonvolatile semiconductor device has a protective film comprised of a silicon nitride film on which is laminated a silicon oxynitride film. The silicon nitride film has a thickness of 1000 Å or more and 2000 Å or less and the silicon oxynitride film has a thickness of about 7000 Å or more. The silicon nitride film and the silicon oxynitride film cooperate to prevent moisture from penetrating into the ultraviolet-erasable nonvolatile semiconductor device. The thickness of the silicon nitride film is set so that the time for erasing data in a nonvolatile semiconductor storage element through irradiation with ultraviolet rays is not increased.