PURPOSE: To realize a semiconductor device provided with a capacitance element whose capacitance and withstand voltage are large.

CONSTITUTION: A semiconductor device 2 has an N-type epitaxial layer 12 being an island region subjected to PN junction isolation by an isolation diffusion layer 13, on the surface side of a P- type semiconductor substrate 11, and has a diffusion layer 14 which is formed by diffusion from the surface side of the layer 12 as far as a buried layer 17 and constitutes a junction capacitance together with the epitaxial layer 12 and the buried layer 17, in the inside of the epitaxial layer 12. The diffusion layer 14 and the isolation diffusion layer 13 are regions which are formed by diffusion at the same time.