In a semiconductor device of the present invention, a MOS transistor is disposed in an elliptical shape. Linear regions in the elliptical shape are respectively used as the active regions, and round regions in the elliptical shape is used respectively as the inactive regions. In each of the inactive regions, a P type diffusion layer is formed to coincide with a round shape. Another P type diffusion layer is formed in a part of one of the inactive regions. These P type diffusion layers are formed as floating diffusion layers, are capacitively coupled to a metal layer on an insulating layer, and assume a state where predetermined potentials are respectively applied thereto. This structure makes it possible to maintain current performance of the active regions, while improving the withstand voltage characteristics in the inactive regions.