A method for manufacturing a semiconductor device includes the steps of forming a first conductive film over a substrate; forming an insulating film over the first conductive film; forming an oxide semiconductor film over the insulating film to overlap with the first conductive film; forming a second conductive film including a metal film containing molybdenum as its main component and a metal film containing copper as its main component over the oxide semiconductor film; and etching the second conductive film by an etchant. At the time of etching the second conductive film by the etchant, the oxide semiconductor film is used as an etching stopper film. In addition, the etchant which can be used for a transistor including the oxide semiconductor film is provided.