When a semiconductor device having an element isolation structure is formed, first, a trench is formed in a wafer from a principal surface of the wafer, and the trench is filled with an insulating film. Then, the back surface of the wafer is polished so that the insulating film is exposed on the back surface. Accordingly, the insulating film penetrates the wafer from the principal surface to the back surface, thereby performing element isolation of the wafer. It is not necessary to use a bonding wafer. Thus, the method for manufacturing the semiconductor device is simplified.