PURPOSE: To enable disposition of a neutron absorber by forming a vacancy inside a basket by disposing accommodation square tubes so that they come into internal contact with a cylindrical main body.

CONSTITUTION: A spent fuel assembly is inserted into an accommodation square tube 2 which is accommodated inside a cylindrical main body 1 of a basket. Gaps (a), (b) and (c) formed between the main body 1 and the outermost square tubes 2 are narrowed, a space thus formed is transferred to the inside of the main body 1 of the basket, and neutron absorbers 3 are disposed in internal gaps including the gaps (a), (b) and (c). Each of these absorbers 3 is so constructed that it has a vacancy 4 inside and that water as a neutron moderator is put therein. By transferring the gaps to the inside of the main body 1 and by making these gaps be neutron trap regions, a neutron multiplication effect can be suppressed without changing the dimensions of the main body 1 and the number of spent fuel accommodation bodies, and thus a contribution can be made to improvement in a subcritical degree.