A multi-layered circuit substrate for a semiconductor device comprises a multi-layered circuit substrate body having first and second surfaces and comprising a plurality of conductive pattern layers integrally laminated one on the other from the first surface to the second surface, so that a plurality of semiconductor device elements can be arranged on the first surface of the substrate body; and a plate member, a rigidity thereof being higher than that of the substrate body, attached to the second surface of the substrate body. A plurality of semiconductor elements can be mounted on the semiconductor element mounting surface defined on the first surface of the substrate body.