The present invention relates to a semiconductor device including a high withstand voltage MOS transistor and a manufacturing method thereof. The semiconductor device according to the present invention includes a MOS transistor in which a second-conductivity type source region is formed on a first-conductivity type semiconductor region, an offset drain region is interconnected to a second-conductivity type drain region and has a concentration lower than an impurity concentration of a drain region, the offset drain region is composed of a portion that does not overlap a first-conductivity type semiconductor region and a portion that overlaps part of the surface of the first-conductivity type semiconductor region and a gate electrode is formed on the surface extending from a channel region between the source region and the offset drain region to part of the offset drain region through a gate insulating film.Thus, there can be obtained an offset drain type MOS transistor having a stable threshold voltage Vth and a low ON-state resistance.