A semiconductor device according to a preferred embodiment of the present invention is a semiconductor device including a main substrate and one or more sub substrates, and the semiconductor device includes first heat generating devices mounted on the sub substrates, sub-substrate heatsinks mounted to the first heat generating devices, and a main-substrate heatsink mounted to the main substrate, wherein the sub-substrate heatsinks and the main-substrate heatsink are secured to each other, such that there is a predetermined positional relationship between the sub substrates and the main substrate.