A thin film transistor includes a gate insulating layer covering a gate electrode, a semiconductor layer in contact with the gate insulating layer, and impurity semiconductor layers which are in contact with part of the semiconductor layer and which form a source region and a drain region. The semiconductor layer includes a microcrystalline semiconductor layer formed on the gate insulating layer and a microcrystalline semiconductor region containing nitrogen in contact with the microcrystalline semiconductor layer. The thin film transistor in which off-current is small and on-current is large can be manufactured with high productivity.