A power MOSFET having a groove for forming a channel improved for shortening the switching time and increasing the dielectric breakdown strength of the gate oxide film is disclosed. The power MOSFET includes a concave structure in which a gate oxide film at a groove bottom is thickened. Namely, since the gate oxide film between a gate electrode and a first conductivity type semiconductor layer is thick, the capacitance of the oxide film therebetween is reduced. Therefore, the input and output capacitance of the gate oxide film can be reduced, and switching loss can be also reduced since the switching time can be shortened. Further, greater dielectric breakdown strength of the gate oxide film can be obtained as a result of the thickened gate oxide film at the groove bottom.