A semiconductor device includes an insulating substrate, semiconductor elements and a cooling device. The cooling device includes a heat radiation substrate, fins, and a cooling case of a box-like shape that accommodates the fins and has a bottom wall and side walls. An introducing port and a discharge port for a cooling liquid are provided diagonally in a pair of side walls provided along the longitudinal direction of the assembly of the fins, among the side walls of the cooling case. A diffusion wall facing the introducing port is provided inside the cooling case.