A semiconductor device has: a radiator plate that is maintained at a predetermined potential; an SOI (Silicon On Insulator) chip mounted on the radiator plate; and thermal grease applied to an interface between the radiator plate and the SOI chip. The SOI chip has: a first silicon substrate forming a circuit element part; a second silicon substrate facing the radiator plate; and an insulating film formed between the first silicon substrate and the second silicon substrate. The first silicon substrate and the second silicon substrate are electrically connected to each other. The thermal grease is conductive and electrically connects the second silicon substrate and the radiator plate.