A semiconductor device comprising: an insulating substrate; a semiconductor element carried on the insulating substrate; and a cooling device that cools the semiconductor element, wherein the cooling device includes: a heat radiation substrate joined to the insulating substrate; an assembly of a plurality of fins provided in a substantially rectangular region on a surface of the heat radiation substrate on a side opposite that of a surface joined to the insulating substrate; and a cooling case of a box-like shape that accommodates the fins and has a bottom wall and four side walls provided at a circumferential edge of the bottom wall, wherein a first side wall (23b1) and a second side wall (23b2) are provided on opposite ends of the bottom wall and parallel to each other, wherein the first side wall (23b1) and the second side wall (23b2) are among the side walls on the long side, and wherein a longitudinal direction of the cooling case matches a longitudinal direction of the assembly of the fins; an introducing port and a discharge port for a cooling medium are provided in the first side wall (23b1) and the second side wall (23b2), respectively, at positions shifted in mutually opposite directions with respect to a central line (cl) bisecting the first side wall (23b1) and the second side wall (23b2); and a diffusion wall causing the cooling medium introduced from the introducing port to diffuse along the first side wall (23b1) is provided inside the cooling case as an ascending inclined surface formed between a bottom side of the first side wall (23b1) and the bottom wall. an insulating substrate; a semiconductor element carried on the insulating substrate; and a cooling device that cools the semiconductor element, wherein the cooling device includes: a heat radiation substrate joined to the insulating substrate; an assembly of a plurality of fins provided in a substantially rectangular region on a surface of the heat radiation substrate on a side opposite that of a surface joined to the insulating substrate; and a cooling case of a box-like shape that accommodates the fins and has a bottom wall and four side walls provided at a circumferential edge of the bottom wall, wherein a first side wall (23b1) and a second side wall (23b2) are provided on opposite ends of the bottom wall and parallel to each other, wherein the first side wall (23b1) and the second side wall (23b2) are among the side walls on the long side, and wherein a longitudinal direction of the cooling case matches a longitudinal direction of the assembly of the fins; an introducing port and a discharge port for a cooling medium are provided in the first side wall (23b1) and the second side wall (23b2), respectively, at positions shifted in mutually opposite directions with respect to a central line (cl) bisecting the first side wall (23b1) and the second side wall (23b2); and a diffusion wall causing the cooling medium introduced from the introducing port to diffuse along the first side wall (23b1) is provided inside the cooling case as an ascending inclined surface formed between a bottom side of the first side wall (23b1) and the bottom wall. a heat radiation substrate joined to the insulating substrate; an assembly of a plurality of fins provided in a substantially rectangular region on a surface of the heat radiation substrate on a side opposite that of a surface joined to the insulating substrate; and a cooling case of a box-like shape that accommodates the fins and has a bottom wall and four side walls provided at a circumferential edge of the bottom wall, wherein a first side wall (23b1) and a second side wall (23b2) are provided on opposite ends of the bottom wall and parallel to each other, wherein the first side wall (23b1) and the second side wall (23b2) are among the side walls on the long side, and wherein a longitudinal direction of the cooling case matches a longitudinal direction of the assembly of the fins; an introducing port and a discharge port for a cooling medium are provided in the first side wall (23b1) and the second side wall (23b2), respectively, at positions shifted in mutually opposite directions with respect to a central line (cl) bisecting the first side wall (23b1) and the second side wall (23b2); and a diffusion wall causing the cooling medium introduced from the introducing port to diffuse along the first side wall (23b1) is provided inside the cooling case as an ascending inclined surface formed between a bottom side of the first side wall (23b1) and the bottom wall.