To solve a problem of a large influence on practical use of an element substrate such as SiC for a high-voltage driver element, which is caused by a large number of crystal faults and a large amount of leakage current.

In a compound semiconductor of a present embodiment, by forming a single-crystal compound semiconductor such as a SiC film on an insulation film on a substrate to form a MOSFET as an element substrate, an electric field from the substrate, and an electric field from a conductor formed in a groove which is formed as a trench in a part of the single-crystal semiconductor and which has oxide films on side walls, and an electric field from a field plate provided in a surface layer of the element through an insulation film ease an electric field applied from a drain to a source of the MOSFET. Accordingly, an influence of a crystal fault of the single-crystal compound semiconductor such as SiC is decreased. And the embodiment achieves a structure which can minimize a P-well by fully utilizing particularities of a trench structure.