A thin film transistor includes a gate electrode; a gate insulating layer which is provided to cover the gate electrode; a semiconductor layer which is provided over the gate insulating layer to overlap with the gate electrode; an impurity semiconductor layer which is partly provided over the semiconductor layer and which forms a source region and a drain region; and a wiring layer which is provided over the impurity semiconductor layer, where a width of the source region and the drain region is narrower than a width of the semiconductor layer, and where the width of the semiconductor layer is increased at least in a portion between the source region and the drain region.