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 in-oxidiz-able gas atmosphere, a silicon nitride layer (14) segregates out at the interface of the silicon substrate (10) and the silicon oxide film (13). After this, the unnecessary silicon oxide film (13) on the silicon nitride layer (14) is removed, and a silicon oxide layer (15) is formed beneath the exposed silicon nitride layer (14) with oxygen passing through the exposed silicon nitride layer (14). Whereby, a gate electrode (16) is formed on the gate insulating film consisting of the silicon nitride layer (14) and the silicon oxide layer (15).