In the substrate and the epitaxial layer, isolation regions are formed to divide the substrate and the epitaxial layer into a plurality of element formation regions. Each of the isolation regions is formed by connecting first and second P type buried diffusion layers with a P type diffusion layer. By disposing the second P type buried diffusion layer between the first P type buried diffusion layer and the P type diffusion layer, a lateral diffusion width of the first P type buried diffusion layer is reduced. This structure allows a formation region of the isolation region to be reduced in size.