A semiconductor device includes: a nitride group semiconductor functional layer including a second nitride group semiconductor region on a first nitride group semiconductor region where a two-dimensional carrier gas layer is made, the second nitride group semiconductor region functioning as a barrier layer; a first main electrode electrically connected to one end of the two-dimensional carrier gas layer; a second main electrode electrically connected to the other end of the two-dimensional carrier gas layer; and a metal oxide film placed between the first and second main electrodes, electrically connected to the first main electrode, the first main electrode extends over an upper surface of the metal oxide film.