Abstract: A high breakdown voltage semiconductor device includes: an n? type region (101) surrounded by a p? well region (102) on a p? type silicon substrate (100); a drain n+ region (103) connected to a drain electrode (120); a p base region (105) formed so as to surround the drain n+ region (103); a source n+ region (114) formed in the p base region (105); and a p? region (131) for isolating the n? type region (101) into an n? type region (101a) including the drain n+ region (103), and an n? type region (101b) not having the drain n+ region (103). The n? type region (101b) is connected to the drain electrode (120) or the drain n+ region (103) via an n offset region (104) or a polysilicon (304) which is a high resistance element.