A semiconductor device includes a semiconductor layer opposing to a bottom surface and a side surface of a gate electrode. An insulation film is provided between the bottom surface of the gate electrode and the semiconductor layer and between the side surface of the gate electrode and the semiconductor layer. A first conduction-type drain layer is provided in the semiconductor layer on a side of an end part of one of the bottom surface and the side surface of the gate electrode. A second conduction-type source layer is provided in the semiconductor layer opposing to the other one of the bottom surface and the side surface of the gate electrode. A second conduction-type extension layer is provided in the semiconductor layer opposing to a corner part between the side surface and the bottom surface of the gate electrode and has a lower impurity concentration than that of the source layer.