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 .