A method for fabricating a semiconductor device, includes forming a silicon nitride film on a base body, forming a silicon film on said silicon nitride film, forming at least one groove extending from said silicon film to inside of said base body, forming by high-density plasma-enhanced chemical vapor deposition a silicon-containing dielectric film in said groove and on said silicon film in such a way that a silicon-rich layer is formed at a height position spaced apart from said base body within said groove, said silicon-rich layer being higher in silicon content than remaining silicon-containing dielectric film, removing by etching a portion of said silicon-containing dielectric film above said silicon film and a portion of said remaining silicon-containing dielectric film above said silicon-rich layer, if any, and after having removed said silicon-containing dielectric film, removing by etching said silicon-rich layer and said silicon film.