In a conventional semiconductor device, protection of a to-be-protected element from a surge voltage is difficult because the to-be-protected element is turned on before a protection element due to variations in manufacturing conditions. In a semiconductor device of the present invention, a protection element and a MOS transistor have part of their structures formed under common conditions. N type diffusion layers of the protection element and the MOS transistor are formed in the same process, while the N type diffusion layer of the protection element has a larger diffusion width than the N type diffusion layer of the MOS transistor. With this structure, when a surge voltage is applied to an output terminal, the protection element is turned on before the MOS transistor, and thereby the MOS transistor is protected from an avalanche current.