In a semiconductor device, a lightly doped second semiconductor layer of a first conductive type is joined with a heavily doped first semiconductor layer of the first conductive type. A power transistor having a first conductive type channel and a transistor are formed in surface regions of the second semiconductor layer, respectively. A first diffusion layer of a second conductive type is formed in a surface region of the second semiconductor layer to provide a boundary between the power transistor and the transistor. The first semiconductor layer functions as a drain of the power transistor. The first diffusion layer region is set to the same voltage as that of the drain.