In a semiconductor device having a heat radiation plate, the tips of inner leads connected to a semiconductor chip have a lead width w and a lead thickness t, the width being less than the thickness. The inner leads are secured to the heat radiation plate. Fastening the inner leads to the heat radiation plate supports the latter and eliminates the need for suspending leads. A lead pitch p, the lead width w and lead thickness t of the inner lead tips connected to the semiconductor chip have the relations of w