In a semiconductor device, for example, a MOS transistor, of the present invention, a P type diffusion layer as a back gate region is formed in an N type epitaxial layer. An N type diffusion layer as a source region is formed in the P type diffusion layer. The P type diffusion layer is formed to have an impurity concentration peak deeper than the N type diffusion layers. This structure reduces a resistance value of a base region of a parasitic transistor, suppresses an increase in an electric potential of a base region in the MOS transistor, and thereby prevents the parasitic transistor from operating. Moreover, a breakdown voltage characteristic of the MOS transistor, which might be deteriorated by the operation of the parasitic transistor, is improved.