To give the distribution of neutron absorber amount in axial and circumferential directions so as to have sufficiently high reactivity worth also in a core with MOX fuel and operate the core stably by making the reactivity of the upper region where neutron absorber of metal steel pipe is charged be higher than the lower region.

In the cross shape projection of tierod 2, a metal sheath 5 with U-shape cross section is connected to be a blade 6. In this blade 6, a plurality of poison tubes 19 charged with neutron absorber B4C powder 14 in stainless steel tube are continuously arranged and contained. To the poison tubes 19, high reactivity worth is given in the upper region (a) in the core where neutron irradiation is high to be control rods 18. The region (a) with higher reactivity is the upper 1/4 of the axial length where the neutron absorber B4C is charged and it is attained by raising the concentration of B4C in this region.