According to one embodiment, a thin-film transistor includes a first insulating film, an oxide semiconductor layer provided on the first insulating film and a second insulating film provided on the oxide semiconductor layer, and at least one of the first insulating film and the second insulating film includes a first region in contact with the oxide semiconductor layer and a second region further distant from the oxide semiconductor layer than the first region, and the second region has an argon concentration higher than that of the first region.