A thin film transistor (100) of the present invention is provided with a gate insulating layer (34), which is formed of an oxide containing lanthanum and zirconium (provided that this oxide may include impurities, and hereinafter the same shall apply with respect to oxides), between a gate electrode (20) and a channel (44). The channel (44) is formed of an oxide for channels, said oxide being composed of: a first oxide that contains indium and zinc, while containing zirconium (Zr) at an atomic ratio of from 0.015 to 0.075 (inclusive) when the indium contained therein is taken as 1; a second oxide that contains indium, while containing zirconium (Zr) at an atomic ratio of from 0.055 to 0.16 (inclusive) when the indium (In) contained therein is taken as 1; or a third oxide that contains indium, while containing lanthanum at an atomic ratio of from 0.055 to 0.16 (inclusive) when the indium contained therein is taken as 1.