A semiconductor device includes a base; a gate electrode to which a gate voltage is applied; a source electrode and a drain electrode through which an electric current is generated according to the gate voltage being applied to the gate electrode; a semiconductor layer made of an oxide semiconductor; and a gate insulating layer inserted between the gate electrode and the semiconductor layer. The semiconductor layer includes a channel-forming region and a non-channel-forming region; the channel-forming region is in contact with the source electrode and the drain electrode, and the non-channel-forming region is in contact with the source electrode and the drain electrode.