To provide a technique for accurately forming a drain diffusion layer in an offset region.

In a method of manufacturing a semiconductor apparatus, a silicon oxide film 12, a polysilicon film 13, and a silicon nitride film 14 are deposited on the top surface of an epitaxial layer 5. Openings 21 are formed on the polysilicon film 13 and the silicon nitride film 14 to form a LOCOS oxide films 22 therein. And then, a p-type diffusion layer 18 is formed using the openings 21 by ion implantation according to a self-aligning technique. Thereafter, the LOCOS oxide films 22 are formed in the openings 21. This manufacturing method makes it possible to form, in the offset region, the p-type diffusion layer used as a drain region with high accuracy of position.