A radioactive substance storage container 100 includes a body 10 and a lid 10 having a sealed structure, and a basket 5 housed in the body 1 to hold fuel assemblies. The basket 5 includes a neutron shielding material. The radioactive substance storage container 100 includes a subcritical-performance reinforcing member 20 including a neutron shielding material and located between an inner periphery of the body 1 and an outer periphery of the basket 5.