Abstract: A method for forming a field oxide film in a semiconductor device comprises the steps of sequentially forming a pad oxide film and a first buffer silicon nitride film on a silicon substrate, and then forming a first patterned mask on the first buffer silicon nitride film. Subsequently, the resulting exposed part of the first buffer silicon nitride film is etched to expose a portion of the pad oxide film. The first patterned mask is them removed. A buffer oxide film is formed on the resulting exposed part of the pad oxide film and the etched first buffer silicon nitride film. Then, a second buffer silicon nitride film and a second patterned mask is sequentially formed on the buffer oxide film, followed by etching of the resulting exposed part of the second buffer silicon nitride. The second patterned mask is then removed, followed by a formation of a field oxide film by thermal oxidation on the resulting structure. The second buffer silicon nitride is then removed.