An object of the present invention is to achieve improvement in performance of a thin film transistor including an oxide as a gate insulating layer, or simplification and energy saving in the processes of producing such a thin film transistor. A thin film transistor (100) of the present invention includes a first oxide layer (possibly containing inevitable impurities) (32) consisting of lanthanum (La) and tantalum (Ta), which has a surface (32a) formed after a precursor layer obtained from a precursor solution as a start material including a precursor containing lanthanum (La) and a precursor containing tantalum (Ta) as solutes is exposed to a hydrochloric acid vapor, between a gate electrode (20) and a channel (52). Moreover, in the thin film transistor, the surface (32a) of the first oxide layer (32) is in contact with the channel (52).