In a pretreatment process, a silicon oxide film (13) with nitrogen content is formed on a semiconductor substrate (10). In a segregation process executing heat treatment in an inert gas atmosphere, a silicon nitride layer (14) segregates out at the interface of the silicon substrate (10) and the silicon oxide film (13). In a high dielectric film forming process, the unnecessary silicon oxide film (13) on the silicon nitride layer (14) is removed, a high dielectric oxide layer (15) is formed on the exposed silicon nitride layer (14). Whereby, a gate electrode (16) consisting of the silicon nitride layer (14) and the high dielectric oxide layer (15) is formed.