In a semiconductor device, a gate silicon dioxide layer is formed within a trench of a semiconductor wafer. A first gate electrode is formed on a sidewall of the trench of the semiconductor wafer via the gate silicon dioxide layer. An insulating layer is formed on a bottom of the trench of the semiconductor wafer via the gate silicon dioxide layer and surrounded by the first gate electrode. The insulating layer excludes silicon dioxide and has different etching characteristics from those of silicon dioxide. A second gate electrode is buried in the trench of the semiconductor wafer, and is in contact with the first gate electrode and the insulating layer.