<p>PURPOSE: A method for manufacturing a semiconductor device is provided to prevent the deterioration of an operation property of the semiconductor device due to a key hole by removing the key hole on an insulation layer which fills a second trench. CONSTITUTION: A trench(37) is formed by selectively etching a substrate. The trench is comprised of a first trench(37A) and a second trench(37B). An insulation layer is formed to fill the second trench by a thermal oxidation process. The linewidth of the insulation layer is smaller than the linewidth of the first trench. A gate insulation layer is formed along the surface of a structure including the insulation layer. A gate electrode is formed on the gate insulation layer to fill the rest trench.</p>
