Abstract: A control rod has four blades, each of which has a rectangular cross section in a plane perpendicular to an axis of the control rod, disposed so as to form a cross-shaped cross section in the plane perpendicular to the axis. The blades include a plurality of aligned square tubes having a square cross section in a plane perpendicular to the axis and including a neutron absorber filling hole filled with neutron absorber, frame plates disposed parallel to the aligned square tubes in a direction perpendicular to a width direction of the blade, and on one side end and another side end of the blade in the width direction, respectively, and cover plates disposed along the width direction and sandwiching the aligned square tubes. Each blade has the aligned square tubes, the frame plates and the cover plates joined together as a single-piece construction by Hot Isostatic Pressing diffusion bonding.