Abstract: A compound semiconductor device includes a compound semiconductor layer in which a two-dimensional carrier gas layer is formed, the compound semiconductor layer including a carrier travel layer and a carrier supply layer; first and second main electrodes, which are arranged apart from each other on the compound semiconductor layer, and are ohmically connected to the two-dimensional carrier gas layer; a metal oxide semiconductor film arranged on the compound semiconductor layer between the first main electrode and the second main electrode; and a control electrode arranged on the metal oxide semiconductor film, the control electrode including a titanium film that contacts the metal oxide semiconductor film or a titanium-containing compound film that contacts the metal oxide semiconductor film.