In a semiconductor device including a semiconductor element that produces heat and a substrate on which the semiconductor element is mounted, functions of the substrate are divided between a heat dissipating substrate and a wiring substrate. The heat dissipating substrate has a relatively high thermal conductivity, and includes principal surfaces defined by electric insulators, one of which is provided with an outer conductor located thereon. The wiring substrate is mounted on the upper principal surface of the heat dissipating substrate, has a thermal conductivity lower than that of the heat dissipating substrate, and includes a wiring conductor made mainly of silver or copper and located inside the wiring substrate, the wiring conductor being electrically connected to the outer conductor. The semiconductor element is mounted on the upper principal surface of the heat dissipating substrate and disposed in a through hole of the wiring substrate.