A semiconductor device capable of improving the driving power and a manufacturing method therefor are provided. In a semiconductor device, a gate structure formed by successively stacking a gate oxide film and a silicon layer is arranged over a semiconductor substrate. An oxide film is arranged long the lateral side of the gate structure and another oxide film is arranged along the lateral side of the oxide film and the upper surface of the substrate. In the side wall oxide film comprising these oxide films, the minimum value of the thickness of the first layer along the lateral side of the gate structure is less than the thickness of the second layer along the upper surface of the substrate.