A thin film transistor whose threshold voltage can be controlled and which has a favorable switching characteristic is provided. The thin film transistor includes a first gate electrode layer; a semiconductor layer; a first gate insulating layer provided between the first gate electrode layer and the semiconductor layer; source electrode and drain electrode layers which are provided over the semiconductor layer; a conductive layer covered by the first gate insulating layer and the semiconductor layer and provided so as to overlap with part of the first gate electrode layer; a second gate insulating layer provided so as to cover at least a back channel portion of the semiconductor layer; and a second gate electrode layer provided over the second gate insulating layer so as to overlap with the back channel portion of the semiconductor layer.