A method for manufacturing a semiconductor device which includes steps of forming a dummy layer on a semiconductor substrate, forming a groove 12 in the semiconductor substrate while using the dummy layer as a mask, forming a tunnel insulating film and a trap layer to cover an inner surface of the groove and the dummy layer, eliminating the trap layer formed above the upper surface and at the sides of the dummy layer, and forming a top insulating film to cover a remaining trap layer and the exposed tunnel insulating film.