A semiconductor device includes a semiconductor element, a heat sink, a core layer, a build-up layer, and a heat dissipation insulating layer. The heat sink has a front surface on which the semiconductor element is mounted and a back surface opposite to the front surface. The core layer has one surface and an opposite surface opposite to the one surface and is formed with a through-hole extending from the one surface to the opposite surface and in which the heat sink is disposed. The build-up layer is disposed to face the front surface and the one surface, and includes a wiring that includes a portion electrically connected to the semiconductor element. The heat dissipation insulating layer is in contact with the opposite surface of the core layer and the back surface of the heat sink.