According to the method of producing a semiconductor device, the substrate is provided with an opening formed at a substantially central position, interconnections and joining parts. The heat spreading plate has a fixed portion fixed to the substrate, a stage portion caved with respect to the fixed potion and connecting portions connecting the fixed portion and the stage portion. The heat spreading plate is fixed by positioning the stage portion at a position opposing the opening, then the heat spreading plate is welded to the substrate and the semiconductor chip is mounted on the stage portion through the opening. Then the semiconductor chip and interconnections formed on the substrate are electrically connected and sealing resin is formed on both sides of the heat spreading plate such that at least the semiconductor chip is sealed.