The present invention provides a method for manufacturing a semiconductor device, comprising the steps of preparing a substrate having a quartz support substrate and a silicon layer, forming a base or substrate silicon oxide film over the entire upper surface of the silicon layer, forming a silicon nitride film over the entire upper surface of the substrate silicon oxide film by a plasma CVD method, patterning the silicon nitride film thereby to form a mask pattern having a circumferential exposure portion that exposes the substrate silicon oxide film in a circumferential area, a first opening pattern that exposes the substrate silicon oxide film in an element isolation area, and a second opening pattern that exposes the substrate silicon oxide film within a peripheral area, and thermally oxidizing the substrate using the mask pattern as a mask thereby to form an element isolation structure portion in the element isolation area.