In a semiconductor device of the present invention, an N type epitaxial layer is formed on a P type single crystal silicon substrate. In the substrate and the epitaxial layer, an N type buried diffusion layer is formed on a P type buried diffusion layer. With this structure, an upward expansion of the P type buried diffusion layer is checked and a thickness of the epitaxial layer can be made small while maintaining the breakdown voltage characteristics of a power semiconductor element. Accordingly, a device size of a control semiconductor element can be reduced.