A study was conducted for the flexural retrofit of an old apartment house composed of a reinforced concrete (RC) shear wall structure. For the shear wall, a vertical retrofit was performed to both ends of the wall targeting the slender wall to improve the flexural performance. The retrofit materials were steel plate, epoxy, and non-shrink grout, and they were connected to the existing shear wall using post-installed chemical anchors. The concrete at the wall ends was broken and retrofitted as much as necessary to maintain the shear wall’s length. The points to be noted are the fracture of the welded part of the flexural retrofit and the chemical anchor’s pulling. After the real scale specimen was fabricated by simulating the existing shear wall, the retrofit effect was experimentally verified by breaking the wall ends, retrofitting the ends, and applying repeated cyclic lateral loads. A total of three specimens were fabricated and retrofitted using steel plates and steel tubes. Since this experiment evaluated the flexural performance, the experiment was performed with a cantilever setting. The flexural retrofit effect using steel was analyzed through the load–displacement relationship, stiffness degradation, and energy dissipation, and through this, the bond of the retrofit and the behavior of the wall were verified.