An object is to provide a thin film transistor using an oxide semiconductor layer, in which contact resistance between the oxide semiconductor layer and source and drain electrode layers is reduced and electric characteristics are stabilized. The thin film transistor is formed in such a manner that a buffer layer including a high-resistance region and low-resistance regions is formed over an oxide semiconductor layer, and the oxide semiconductor layer and source and drain electrode layers are in contact with each other with the low-resistance region of the buffer layer interposed therebetween.