A semiconductor device comprising a semiconductor substrate having an upper main surface and a lower main surface, said semiconductor substrate including: a drain layer of a first conductivity type formed in said upper main surface; a main base region of a second conductivity type selectively formed in said drain layer to be shallower than said drain layer and exposed to said upper main surface; an underpad base region of a second conductivity type selectively formed in said drain layer to be shallower than said drain layer and exposed to said upper main surface, said underpad base region being isolated from said main base region and being deeper than said main base region; and a source region of a first conductivity type selectively formed in said main base region to be shallower than said main base region and exposed to said upper main surface, said semiconductor device further comprising: a first main electrode connected to said main base region and said source region and not connected to said underpad base region; a gate electrode opposed to a channel region in said main base region interposed between said drain layer and said source region with a gate insulating film provided therebetween; a conductive gate pad provided on an insulating layer to be opposed to an exposed surface of said underpad base region in said upper main surface with said insulating layer interposed therebetween and connected to said gate electrode; and a second main electrode connected to said lower main surface, characterized in that said drain layer is interposed between said main base region and said underpad base region and said main base region is immediately adjacent to said drain layer and said drain layer is immediately adjacent to said underpad base region. a drain layer of a first conductivity type formed in said upper main surface; a main base region of a second conductivity type selectively formed in said drain layer to be shallower than said drain layer and exposed to said upper main surface; an underpad base region of a second conductivity type selectively formed in said drain layer to be shallower than said drain layer and exposed to said upper main surface, said underpad base region being isolated from said main base region and being deeper than said main base region; and a source region of a first conductivity type selectively formed in said main base region to be shallower than said main base region and exposed to said upper main surface, said semiconductor device further comprising: a first main electrode connected to said main base region and said source region and not connected to said underpad base region; a gate electrode opposed to a channel region in said main base region interposed between said drain layer and said source region with a gate insulating film provided therebetween; a conductive gate pad provided on an insulating layer to be opposed to an exposed surface of said underpad base region in said upper main surface with said insulating layer interposed therebetween and connected to said gate electrode; and a second main electrode connected to said lower main surface, characterized in that said drain layer is interposed between said main base region and said underpad base region and said main base region is immediately adjacent to said drain layer and said drain layer is immediately adjacent to said underpad base region. a first main electrode connected to said main base region and said source region and not connected to said underpad base region; a gate electrode opposed to a channel region in said main base region interposed between said drain layer and said source region with a gate insulating film provided therebetween; a conductive gate pad provided on an insulating layer to be opposed to an exposed surface of said underpad base region in said upper main surface with said insulating layer interposed therebetween and connected to said gate electrode; and a second main electrode connected to said lower main surface, characterized in that said drain layer is interposed between said main base region and said underpad base region and said main base region is immediately adjacent to said drain layer and said drain layer is immediately adjacent to said underpad base region.