An object is to provide a thin film transistor including an oxide semiconductor layer, in which a material used for the oxide semiconductor layer and a material used for source and drain electrode layers are prevented from reacting with each other. The source and drain electrode layers provided over a substrate having an insulating surface have a stacked structure of two or more layers. In the stack of layers, a layer which is in contact with an oxide semiconductor layer is a metal layer including a metal element other than a metal element included in the oxide semiconductor layer. An element selected from Sn, Sb, Se, Te, Pd, Ag, Ni, and Cu; an alloy containing any of these elements as a component; an alloy containing any of these elements in combination; or the like is used for a material of the metal layer used.