A method for manufacturing a semiconductor device includes forming a silicon substrate having first and second surfaces, the silicon substrate including no oxide film or an oxide film having a thickness no greater than 100 nm, forming a first oxide film at least on the second surface of the silicon substrate, forming a first film by covering at least the first surface, forming a mask pattern on the first surface by patterning the first film, forming a device separating region on the first surface by using the mask pattern as a mask, forming a gate insulating film on the first surface, forming a gate electrode on the first surface via the gate insulating film, forming a source and a drain one on each side of the gate electrode, and forming a wiring layer on the silicon substrate while maintaining the first oxide film on the second surface.