A semiconductor device includes a cooling plate made of metal, one or more laminated substrates each formed by laminating a circuit board, an insulating board, and a metal board, and one or more first semiconductor elements each made of a wide-band-gap semiconductor and disposed over outer peripheral edge portions of the circuit board. The metal board and the cooling plate are joined by the use of a joining material. As a result, even if temperature rises due to the operation of the one or more first semiconductor elements and heat radiation is not performed properly, the one or more first semiconductor elements operate stably.