A semiconductor device comprising: a first insulating layer including silicon nitride; a first conductive layer including copper over the first insulating layer; a second conductive layer covering the first conductive layer; a second insulating layer including silicon nitride over the first conductive layer and the second conductive layer; a third insulating layer including silicon oxide over the second insulating layer; an oxide semiconductor layer over the third insulating layer; a source electrode and a drain electrode, which are provided over the oxide semiconductor layer; a fourth insulating layer including silicon oxide over the source electrode, the drain electrode and the oxide semiconductor layer; a fifth insulating layer including silicon nitride over the fourth insulating layer; a third conductive layer over the fifth insulating layer with a stacked-layer structure of a first conductive sublayer including an element which has a higher melting point than copper, and of a second conductive sublayer formed over the first conductive sublayer and including copper, wherein the third conductive layer is electrically connected to one of the source electrode and the drain electrode; a sixth insulating layer including silicon nitride over the third conductive layer; and a fourth conductive layer over the sixth insulating layer and electrically connected to the other one of the source electrode and the drain electrode, wherein the first conductive layer does not overlap with the oxide semiconductor layer and is in contact with the second conductive layer, wherein a part of the second conductive layer is extended to overlap with the oxide semiconductor layer and function as a gate electrode of a thin film transistor, wherein the first conductive layer as well as the second conductive layer is a gate wiring, and wherein a melting point of the first conductive layer is smaller than a melting point of the second conductive layer. a first insulating layer including silicon nitride; a first conductive layer including copper over the first insulating layer; a second conductive layer covering the first conductive layer; a second insulating layer including silicon nitride over the first conductive layer and the second conductive layer; a third insulating layer including silicon oxide over the second insulating layer; an oxide semiconductor layer over the third insulating layer; a source electrode and a drain electrode, which are provided over the oxide semiconductor layer; a fourth insulating layer including silicon oxide over the source electrode, the drain electrode and the oxide semiconductor layer; a fifth insulating layer including silicon nitride over the fourth insulating layer; a third conductive layer over the fifth insulating layer with a stacked-layer structure of a first conductive sublayer including an element which has a higher melting point than copper, and of a second conductive sublayer formed over the first conductive sublayer and including copper, wherein the third conductive layer is electrically connected to one of the source electrode and the drain electrode; a sixth insulating layer including silicon nitride over the third conductive layer; and a fourth conductive layer over the sixth insulating layer and electrically connected to the other one of the source electrode and the drain electrode, wherein the first conductive layer does not overlap with the oxide semiconductor layer and is in contact with the second conductive layer, wherein a part of the second conductive layer is extended to overlap with the oxide semiconductor layer and function as a gate electrode of a thin film transistor, wherein the first conductive layer as well as the second conductive layer is a gate wiring, and wherein a melting point of the first conductive layer is smaller than a melting point of the second conductive layer.