A semiconductor device includes: a semiconductor module and a heat dissipation sheet attached to a bottom surface of the semiconductor module, the heat dissipation sheet including: a sheet-shaped first conduction part that has a first main surface bonded to the bottom surface of the circuit substrate, a thermal conductivity of the first conduction part in directions along the first main surface being higher than a thermal conductivity of the first conduction part in a thickness direction; and a sheet-shaped second conduction part that is provided next to the first conduction part at an end of the first conduction part and that has a second main surface continuing from the first main surface, a thermal conductivity of the second conduction part in a thickness direction being higher than a thermal conductivity of the second conduction part in directions along the second main surface.