Semiconductor device includes a semiconductor layer, an insulating film provided on the semiconductor layer and having an opening formed therein, a gate electrode connected to the semiconductor layer through opening, a protection film covering gate electrode, and a Ni oxide film, wherein the insulating film has a first surface on the semiconductor layer side and a second surface opposite to the first surface, and the gate electrode has a third surface facing the second surface and spaced apart from the second surface and a fourth surface connecting the second surface and the third surface. The gate electrode includes a Ni film constituting the third surface and the fourth surface, and the Ni oxide film covers the Ni film on the third surface and the fourth surface. The protection film covers the third surface and the fourth surface by being placed over Ni oxide film.