A manufacturing method of a semiconductor device according to an embodiment includes: forming a trench for a device isolation area and a semiconductor projection with a first width by etching a semiconductor substrate; forming an oxide film on the trench and the semiconductor projections; forming an insulating layer on the oxide film; exposing the upper surface of the semiconductor projection by polishing the insulating layer and the oxide film; forming a gate insulating layer at a lower region of the semiconductor projection; and etching the insulating layer and the oxide film on the substrate.