A semiconductor device according to the present invention includes an island provided on one surface of a resin substrate, an external terminal provided on the other surface of the substrate, a thermal pad provided on the other surface of the substrate in opposed relation to the island, a heat conduction portion extending through the substrate from the one surface to the other surface to connect the island to the thermal pad in a thermally conductive manner, and a solder resist portion provided on the other surface of the substrate and having a heat dissipation opening which defines a gap with respect to an outer periphery of the thermal pad and a terminal opening which exposes the external terminal.