A method for manufacturing a vertical MOS transistor comprising forming a protrusion-like region, forming a silicon oxide film on an exposed surface of the protrusion-like region and a surface of the silicon semiconductor substrate, increasing a film thickness of at least the silicon oxide film on the silicon semiconductor substrate by thermal oxidation to form a first insulating film, forming a lower impurity diffusion region, removing the silicon oxide film to expose a silicon side of the protrusion-like region, thermally oxidizing the silicon side to form a second insulating film having a thinner film thickness than a film thickness of the first insulating film, forming a gate electrode over a side of the protrusion-like region, and forming an upper impurity diffusion region.