Abstract: In a method for forming an isolation region in a semiconductor device, after forming a first oxide film and a silicon film on a semiconductor substrate, an oxidation-blocking film is formed on the silicon film. Then, a high-temperature heat treatment process is performed in a nitrogenous atmosphere. The oxidation-blocking film is selectively etched to form an opening, and a thermal oxidation process is performed to form a thermal oxide film in the opening. A bird's beak between the oxidation-blocking film and the silicon film is suppressed because of the heat treatment in a nitrogenous atmosphere, so that stable isolation characteristics can be secured.