To provide a containment cask for storage or transport of radioactive material, without employing a homogenization treatment. Pouring a molten lead between an inner shell 2 and an intermediate shell 3 to serve as a gamma ray shielding material, allowing the lead to cool, and subsequently, filling either one or both of a first void layer 9a formed at a boundary between the inner shell 2 and the poured lead 5a or a second void layer 9b formed at a boundary between the intermediate shell 3 and the poured lead 5a, using a low melting point metal 10 in a closely adhering state. To provide the cask 1 with a good heat-dissipating effect, by filling the void layers 9a, 9b that prevent the cask 1 from dissipating heat, with the low melting point metal 10 that has a superb thermal conductivity.