A semiconductor device is provided that can be manufactured by a simpler process than a conventional lateral trench power MOSFET for use with an 80V breakdown voltage, and which has a lower device pitch and lower on-state resistance per unit area than a conventional lateral power MOSFET for use with a lower breakdown voltage than 80V. A gate oxide film is formed thinly along the lateral surfaces of a trench at a uniform thickness. Then, a gate oxide film is formed along the bottom surface of the trench by selective oxidation so as to be thicker than the gate oxide film on the lateral surfaces of the trench and so as to become progressively thicker from the edge of the bottom surface of the trench toward drain polysilicon.