A TFT 20 includes a gate electrode 21, a gate insulating film 22, a semiconductor layer 23, a source electrode 24, a drain electrode 25, etc. The semiconductor layer 23 is comprised of a metal oxide semiconductor (IGZO), and has a source portion 23a that contacts the source electrode 24, a drain electrode 23b that contacts the drain electrode 25, and a channel portion 23c that is located between the source and drain portions 23a, 23b. A reduced region 30 is formed at least in the channel portion 23c of the semiconductor layer 23, and the reduced region 30 has a higher content of a simple substance of a metal such as In than the remaining portion of the semiconductor layer 23.