A cask comprising: a body for accommodating a basket formed with a plurality of cells where spent fuel assemblies are accommodated: a plurality of heat conductive fins (107, 210) provided around an external periphery of the body; an external cylinder fixed to an external periphery of the heat conductive fins (107, 210), and a neutron absorber provided in a space formed with the heat conductive fins (107, 210), the body and the external cylinder, wherein the external cylinder comprises a plurality of first belt-like members (105a, 216a) and a plurality of second belt-like members (105b, 216b) which are welded together along edges thereof, wherein each one of the first belt-like members and each one of the second belt-like members are arranged in turns to form the external cylinder; one of the heat conductive fins (107, 210) is welded along one side of each of the first belt-like members (105a; 216a) in terms of a circumferential direction of the external cylinder, with a margin left along an edge of said first belt-like member (105a; 216a); another one of the heat conductive fins (107, 210) is welded along the other side of each of the first belt-like members (105a, 216a), in terms of the circumferential direction of the external cylinder, with a margin left along the other edge of said first belt-like member (105a; 216a). a body for accommodating a basket formed with a plurality of cells where spent fuel assemblies are accommodated: a plurality of heat conductive fins (107, 210) provided around an external periphery of the body; an external cylinder fixed to an external periphery of the heat conductive fins (107, 210), and a neutron absorber provided in a space formed with the heat conductive fins (107, 210), the body and the external cylinder, wherein the external cylinder comprises a plurality of first belt-like members (105a, 216a) and a plurality of second belt-like members (105b, 216b) which are welded together along edges thereof, wherein each one of the first belt-like members and each one of the second belt-like members are arranged in turns to form the external cylinder; one of the heat conductive fins (107, 210) is welded along one side of each of the first belt-like members (105a; 216a) in terms of a circumferential direction of the external cylinder, with a margin left along an edge of said first belt-like member (105a; 216a); another one of the heat conductive fins (107, 210) is welded along the other side of each of the first belt-like members (105a, 216a), in terms of the circumferential direction of the external cylinder, with a margin left along the other edge of said first belt-like member (105a; 216a). a plurality of heat conductive fins (107, 210) provided around an external periphery of the body; an external cylinder fixed to an external periphery of the heat conductive fins (107, 210), and a neutron absorber provided in a space formed with the heat conductive fins (107, 210), the body and the external cylinder, wherein the external cylinder comprises a plurality of first belt-like members (105a, 216a) and a plurality of second belt-like members (105b, 216b) which are welded together along edges thereof, wherein each one of the first belt-like members and each one of the second belt-like members are arranged in turns to form the external cylinder; one of the heat conductive fins (107, 210) is welded along one side of each of the first belt-like members (105a; 216a) in terms of a circumferential direction of the external cylinder, with a margin left along an edge of said first belt-like member (105a; 216a); another one of the heat conductive fins (107, 210) is welded along the other side of each of the first belt-like members (105a, 216a), in terms of the circumferential direction of the external cylinder, with a margin left along the other edge of said first belt-like member (105a; 216a).