A semiconductor device includes a semiconductor substrate having a first gate groove having first and second side walls facing to each other. A first gate insulating film covers the first and second side walls. A first gate electrode is disposed on the first gate insulating film and in a lower portion of the first gate groove. A first burying insulating film buries the first gate groove and covers the first gate electrode. A first diffusion region is adjacent to a first upper portion of the first gate insulating film. The first upper portion is positioned on an upper portion of the first side wall of the first gate groove. A second diffusion region is in contact with an upper portion of the second side wall of the first gate groove.