A novel semiconductor device with a transistor using an oxide semiconductor film, in which a conductive film including Cu is used as a wiring or the like, is provided. The semiconductor device includes a first insulating film, an oxide semiconductor over the first insulating film, a gate electrode overlapping the oxide semiconductor with a gate insulating film positioned therebetween, a second insulating film in contact with a side surface of the gate electrode, and a third insulating film in contact with a top surface of the gate electrode. The gate electrode includes a Cu—X alloy film (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, Ti, Zr, Mg, Ca, or a mixture of two or more of these elements).