The thin-film transistor of the present invention has at least a semiconductor layer including: on a substrate, a source electrode, a drain electrode, and a channel region; a gate insulating film; and a gate electrode, wherein the semiconductor layer is an oxide semiconductor layer, and wherein the gate insulating film is amorphous silicon including at least O and N, and the gate insulating film has a distribution of an oxygen concentration in a thickness direction so that the oxygen concentration is high in the side of an interface with an oxide semiconductor layer and the oxygen concentration decreases toward the side of the gate electrode.