A semiconductor device includes an oxide semiconductor layer provided over a substrate having an insulating surface; a gate insulating film covering the oxide semiconductor layer; a first conductive layer and a second conductive layer laminated in this order over the gate insulating film; an insulating film covering the oxide semiconductor layer and a gate wiring including a gate electrode (the first and second conductive layers); and a third conductive layer and a fourth conductive layer laminated in this order over the insulating film and electrically connected to the oxide semiconductor layer. The gate electrode is formed using the first conductive layer. The gate wiring is formed using the first conductive layer and the second conductive layer. A source electrode is formed using the third conductive layer. A source wiring is formed using the third conductive layer and the fourth conductive layer.