A semiconductor device of one embodiment, including the semiconductor layer including a III-V group nitride semiconductor; a groove portion formed in the semiconductor layer; the gate insulating film formed at least on a bottom surface of the groove portion, the gate insulating film being a stacked film of a first insulating film and a second insulating film of which dielectric constant is higher than that of the first insulating film; the gate electrode formed on the gate insulating film; and a source electrode and a drain electrode formed on the semiconductor layer across the gate electrode, in which the second insulating film is selectively formed only under the gate electrode.