A semiconductor device includes; a semiconductor layer mainly made of GaN; a protective film provided to have electrical insulation property and configured to coat the semiconductor layer; and an electrode provided to have electrical conductivity and configured to form a Schottky junction with the semiconductor layer. The protective film includes: a first layer made of Al2O3 and arranged adjacent to the semiconductor layer; a second layer made of an electrical insulation material different from Al2O3 and formed on the first layer; and an opening structure formed to pass through the first layer and the second layer. The electrode is located inside of the opening structure.