According to one embodiment, a semiconductor device includes a base region of a second conductivity type, a drift region of a first conductivity type, an insulating layer, a drain region of the first conductivity type, a gate oxide film, a gate electrode, a first main electrode, and a second main electrode. The base region includes a source region of the first conductivity type. The drift region is adjacent to the base region. The insulating layer is provided from a surface to inside of the drift region. The drain region is provided in the surface of the drift region and opposed to the source region across the base region and the insulating layer. The gate oxide film is provided on a surface of the base region. The gate electrode is provided on the gate oxide film. The first main electrode is connected to the source region. The second main electrode is connected to the drain region. As viewed in a direction perpendicular to the surface of the base region, the source region and at least a part of the drain region extend generally parallel in a line shape, and a length of a portion of the drift region sandwiched between the insulating layer and the base region is shorter in the generally parallel extending direction than in a direction generally perpendicular to the generally parallel extending direction.