A semiconductor device includes a substrate, a circuit element disposed on or above the upper surface of the substrate, an electrode disposed on or above the upper surface of the substrate and connected to the circuit element, and a conductor pillar bump for external connection which is disposed on the substrate and electrically connected to the electrode or the circuit element. The substrate includes a first base and a second base disposed on the first base. The circuit element and the electrode are disposed on the second base. The first base has lower thermal resistance than the second base.