A substrate with which a semiconductor device with excellent electric characteristics and high reliability can be manufactured is provided. An aspect of the invention is a method for manufacturing a substrate for manufacturing a semiconductor device: a first silicon oxide film, a silicon nitride film, and a second silicon oxide film are stacked in this order over a surface of a semiconductor substrate by a thermal CVD method, and then a weakened layer is formed at a given depth of the semiconductor substrate; the semiconductor substrate and a substrate having an insulating surface are arranged to face each other, and the second silicon oxide film provided for the semiconductor substrate and a supporting substrate are bonded to each other; and the semiconductor substrate is separated at the weakened layer by heat treatment, whereby a semiconductor film separated from the semiconductor substrate is left over the substrate having the insulating surface.