a method of fabricating a semiconductor device capable of suppressing defective etching in formation of a deep trench also when the number of polishing steps is reduced is obtained . this method of fabricating a semiconductor device comprises steps of forming a first trench on an element isolation region of a semiconductor substrate , forming a first film consisting of an insulator film to fill up the first trench , forming a second trench larger in depth than the first trench in the first trench , forming an embedded film in the second trench and substantially simultaneously polishing an excess depositional portion of the first film and an excess depositional portion of the embedded film .