A semiconductor device includes: an n-type first well diffusion layer; an n-type second well diffusion layer; a p-type source diffusion layer; a p-type third well diffusion layer; a p-type drain diffusion layer; a gate insulating film; a gate electrode; a device isolation insulating film; and a buffer layer. The buffer layer is formed between the first well diffusion layer and the third well diffusion layer to be in contact with an end of the third well diffusion layer opposing the source diffusion layer, and extends from immediately below the gate insulating film to a position deeper than a peak of curvature of impurity concentration distribution of the third well diffusion layer. The buffer layer has an impurity concentration lower than an impurity concentration in the third well diffusion layer.