Heat conductive fins (107) are welded along both sides of a belt-like member (105a) with a margin left along edges of the both sides, thereby to form a unit (105c). The unit (105c) is welded to a body (101) with the heat conductive fins (107) from the outside of each fin. Next, another belt-like member (105b) is covered over a gap between adjacent belt-like members (105a, 105a), and these members are welded from the outside. With this arrangement, there is no need to weld in a narrow and long space and the entire welding step is carried out from the outside, which makes it easier to assemble a cask.