A semiconductor device manufacturing method includes the steps of: successively forming, on a semiconductor substrate, a gate insulating film and first and second dummy sections stacked in this order; forming a notch section by processing the gate insulating film and the first and second dummy gate sections into a previously set pattern and making the first dummy gate section move back in the gate length direction relative to the second dummy gate section; forming a side wall of an insulating material in a side part of each of the gate insulating film and the first and second dummy gate sections and embedding the notch section therewith; removing the first and second dummy gate sections to leave the gate insulating film and the notch section in the bottom of a removed portion; and forming a gate electrode made of a conductive material by embedding the removed portion with the conductive material.