A semiconductor device includes a pair of power chips, an IC chip, a plurality of leads one of which having a die pad on which the power chips are mounted and another one having a die attach portion on which the IC chip is mounted, a resin sheet firmly adhered to one side of the die pad, and a resin casing made by molding operation to encapsulate the power chips, the IC chip and the resin sheet by a resin in such a manner that one surface of the resin sheet opposite the die pad is exposed to the exterior of the resin casing. The resin casing has a groove formed in one surface opposite the exposed surface of the resin sheet, the groove extending parallel to the resin sheet and perpendicular to a runner through which the resin was supplied in the molding operation.