A control rod has blades each of which includes a neutron absorption region in which neutron absorbers each loaded to the core of a reactor from below and made of absorption nuclear chain type neutron absorber are disposed. The neutron absorption region is divided into two, upper and lower, regions in an axial direction; the boundary between the upper and lower regions is positioned within the range of from 3/8 to 5/8 of the full length of the neutron absorption region in the axial direction from the lower end of the neutron absorption region; and the quantity of the absorption nuclear chain type neutron absorber contained in the lower region in the section perpendicular to the axial direction is smaller than the quantity of the absorption nuclear chain type neutron absorber contained in the upper region in the section perpendicular to the axial direction.