A cooling apparatus for semiconductor chips includes radiation fins formed on the opposite surface of metal base opposite to the surface of metal base, to which an insulator base board mounting semiconductor chips thereon, is disposed. The radiation fins, such as sheet-shaped fins having different lengths are arranged such that the surface area density of the fins becomes higher in the coolant flow direction, whereby the surface area density is the total surface area of radiation fins on a unit surface area of the metal base. As a result, the temperatures of semiconductor chips arranged along the coolant flow direction are closer to each other.