A method for manufacturing a semiconductor device including a transistor with stable electrical characteristics or high reliability is provided. A gate insulating film is formed over a gate electrode; an oxide semiconductor layer is formed over the gate insulating film; an oxide layer is formed over the oxide semiconductor layer to form a stacked-layer oxide film including the oxide semiconductor layer and the oxide layer; the stacked-layer oxide film is processed into a predetermined shape; a conductive film containing Ti is formed over the stacked-layer oxide film; the conductive film is etched to form source and drain electrodes; and regions in the oxide layer in contact with the source and drain electrodes are heat treated so as to have a low resistivity.