the present invention is directed to a method of manufacturing a trench type semiconductor element isolation structure including the steps of : forming a silicon oxide film on a silicon substrate and forming a silicon nitride film on the silicon oxide film ; forming a groove penetrating the silicon nitride film and the silicon oxide film , said groove reaching an interior of the silicon substrate ; forming a thermal oxide film on an inner wall of said groove ; depositing an oxide in said groove ; subjecting said oxide to a polishing treatment with the silicon nitride film used as a stopper layer , so that a part of the insulator is removed ; etching the oxide by a predetermined amount of said oxide after completing the step ; etching the silicon nitride film after completing the step ; and etching the silicon oxide film after completing the step .