Provided is a semiconductor device which includes a metal oxide semiconductor (MOS) transistor having high driving performance and high withstanding voltage with a thick gate oxide film. In the local oxidation-of-silicon (LOCOS) offset MOS transistor having high withstanding voltage, in order to prevent a gate oxide film (6) formed on a channel formation region (7) from being etched at a time of removing the gate oxide film (6) with a polycrystalline silicon gate electrode (8) being used as a mask to form a second conductivity type high concentration source region (4) and a second conductivity type high concentration drain region (5), a source field oxide film (14) is formed also on a source side of the channel formation region (7), and in addition, a length of a second conductivity type high concentration source field region (13) is optimized. Accordingly, it is possible to obtain a MOS transistor having high driving performance and high withstanding voltage with a thick gate oxide film.