method of manufacturing a semiconductor device capable of manufacturing a miniaturized semiconductor device is provided. The method of manufacturing a semiconductor device according to an embodiment includes the steps of: preparing a semiconductor substrate having a first surface and a second surface which is an opposite surface of the first surface; forming a hard mask having an opening on the first surface; forming a gate trench extending toward the second surface on the first surface using the hard mask as a mask; widening the width of the opening; filling the opening with an interlayer insulating film; and forming a contact hole in the interlayer insulating film by removing the hard mask.