A semiconductor device includes an oxide semiconductor layer, a source electrode and a drain electrode in contact with the oxide semiconductor layer, a gate electrode overlapping with the oxide semiconductor layer, and a gate insulating layer between the oxide semiconductor layer and the gate electrode, in which the source electrode or the drain electrode comprises a first conductive layer and a second conductive layer having a region which extends beyond an end portion of the first conductive layer in a channel length direction and which overlaps with part of the gate electrode, in which a sidewall insulating layer is provided over the extended region of the second conductive layer, and in which the sidewall insulating layer comprises a stack of a plurality of different material layers.