Disclosed is an oxide for a semiconductor layer of a thin-film transistor, said oxide being excellent in the switching characteristics of a thin-film transistor, specifically enabling favorable characteristics to be stably obtained even in a region of which the ZnO concentration is high and even after forming a protective film and after applying stress. The oxide is used in a semiconductor layer of a thin-film transistor, and the aforementioned oxide contains Zn and Sn, and further contains at least one element selected from group X consisting of Al, Hf, Ta, Ti, Nb, Mg, Ga, and the rare-earth elements.