An n-type buried diffusion layer is formed on the surface layer of the prescribed area of a p-type silicon substrate, and a p-type first high-concentration isolation diffusion layer is formed in the silicon substrate so as to surround the buried diffusion layer. An n-type epitaxial layer is formed on the silicon substrate, the buried diffusion layer, and the first high-concentration isolation diffusion layer. A p-type second high-concentration isolation diffusion layer is formed in the epitaxial layer on the first high-concentration isolation diffusion layer. A p-type low-concentration isolation diffusion layer for isolating the epitaxial layer into a plurality of island regions is formed in the epitaxial layer on the second high-concentration isolation diffusion layer.