PURPOSE: To contrive the reduction of weight by setting a neutron absorbing plate on the outer face of a thin plate structure cell set in parallel with an axial direction while setting a water cell between structural cells and liquid- tightly mounting upper and lower plates between the opening parts of upper and lower ends of an outer cylinder.

CONSTITUTION: A plurality of thin plate structural cells 9 in which radioactive materials are individually housed are formed in the form of cross section square, neutron absorbing plates 19 are attached to the outer face and set in outer cylinders 14 at intervals in parallel with an axial direction while water cells 12 are set between the cells 9. Further, an upper plate 13 and a lower plate having drain holes are liquid-tightly mounted on upper and lower end opening parts. After radioactive materials are housed in water from the hole of the upper plate 13 into the cells 9, taken out from the water to drain the water in the cells 9 from the holes of a bottom plate and the cells 9, 12 are dried, they are put in a radioactive material transporting vessel to transfer and store. Therefore, cutwater is made good and weight can be reduced.