Provided is a manufacturing method for an offset MOS transistor capable of operating safely even under a voltage of 50 V or higher. In the offset MOS transistor which includes a LOCOS oxide film, the LOCOS oxide film formed in a periphery of a drain diffusion layer, in which a high withstanding voltage is required, is etched, and the drain diffusion layer is formed so as to spread into a surface region of a semiconductor substrate located below a region in which the LOCOS oxide film is thinned. As a result, end portions of the drain diffusion layer are covered by an offset diffusion layer, whereby electric field concentration occurring in a region of a lower portion of the drain diffusion layer can be relaxed.