An object is to provide a semiconductor device which achieves miniaturization as well as suppressing a defect. Further, another object is to provide a semiconductor device which achieves miniaturization as well as keeping favorable characteristics. Is provided a semiconductor device including: a source wiring and a drain wiring each of which include a first conductive layer and a second conductive layer having a smaller thickness than the first conductive layer; an insulating layer which has an opening portion and is provided over the source wiring and the drain wiring; an oxide semiconductor layer which is in contact with part of the second conductive layer of the source wiring or the drain wiring in the opening portion; a gate insulating layer provided over the oxide semiconductor layer; and a gate electrode provided over the gate insulating layer.