A first thermal treatment, which is performed at a temperature within 650 750.degree. C. for 30 240 minutes, and thereafter a second thermal treatment, which is performed at a temperature within 900 1100.degree. C. for 30 120 minutes, are performed as the initial thermal treatments on a semiconductor wafer composed of silicon. Further, before forming a gate insulating film, the temperature is increased to 1000.degree. C. at a temperature increasing rate of 8.degree. C./min in a nitrogen ambient, and a thermal treatment is performed at a temperature of 1000.degree. C. for 30 minutes as a third thermal treatment.