One embodiment of the present invention is a thin film transistor, including: an insulating substrate; a gate electrode, a gate insulating layer and a semiconductor layer including an oxide, these three elements being formed over the insulating substrate in this order, and the gate insulating layer including: a lower gate insulating layer, the lower gate insulating layer being in contact with the insulating substrate and being an oxide including any one of the elements In, Zn or Ga; and an upper gate insulating layer provided on the lower gate insulating layer, the upper gate insulating layer comprising at least one layer; and a source electrode and a drain electrode formed on the semiconductor layer.