A semiconductor device 1 includes an insulating substrate 14, semiconductor elements 12, 13, and a cooling device 20. The cooling device 20 includes a heat radiation substrate 21, fins 22, and a cooling case 23 of a box-like shape that accommodates the fins 22 and has a bottom wall 23a and side walls 23b. An introducing port 23c and a discharge port 23d for a cooling liquid are provided diagonally in a pair of side walls provided along the longitudinal direction of the assembly of the fins 22, among the side walls 23b of the cooling case 23. A diffusion wall 23g facing the introducing port 23c is provided inside the cooling case 23.