According to one embodiment, a thin-film transistor comprises an oxide semiconductor layer formed on a part of a substrate, a first gate insulator film of a silicon dioxide film formed on the oxide semiconductor layer and by the CVD method with a silane-based source gas, a second gate insulator film of a silicon dioxide film formed on the first gate insulator film by the CVD method with a TEOS source gas, and a gate electrode formed on the second gate insulator film.