A semiconductor device includes an insulated circuit board having a lower surface, a heat dissipation base plate having a front surface that includes an arrangement region in which the lower surface of the insulated circuit board is arranged via solder and a solder region in which the solder spreads over the arrangement region, a plating film formed on the front surface of the heat dissipation base plate except for the solder region, and an alloy layer disposed between the solder and the heat dissipation base plate in the arrangement region. The alloy layer contains a solder component that is contained in the solder. In particular, in the semiconductor device, the plating film is formed on an entire surface of the heat dissipation base plate except for an opening region surrounding an outer periphery of the arrangement region where the insulated circuit board is arranged via the solder.