A method of manufacturing a MOS-type semiconductor device capable of increasing the thickness of a gate oxide film and obtaining high gate withstanding power and reduced switching loss without increasing a gate threshold voltage Vth is provided. A p-type well region is selectively formed on one principle surface of a semiconductor substrate having an n-type low impurity concentration layer by using an oxide film as a mask. Subsequently, a resist mask is formed on the surface of the p-type well region so as to be separated from the oxide film mask, and an n+-type source region is selectively formed from the separation portion. Subsequently, the oxide film mask is removed. Then, an oxide film is formed on the surface of the p-type well region, and the oxide film is removed. Subsequently, a gate electrode coated with a gate oxide film is formed on the surface of the semiconductor substrate.