A semiconductor device in which chips are resin-molded, including: frames having front and back surfaces and die pads; power chips mounted on the surfaces of the die pads; an insulation resin sheet having a first and a second surfaces which are opposed against each other, the resin sheet being disposed such that the back surfaces of the die pads contact the first surface of the resin sheet; and a mold resin applied on the first surface of the resin sheet so as to seal up the power chips. The thermal conductivity of the resin sheet is larger than that of the mold resin.