We have developed a new shear-key using a steel disk and an anchor bolt in order to use in joints of seismic retrofitting. It was already obvious that the shear-key had higher shear strength and stiffness than post-installed anchors. Outside-frame method is one of seismic retrofitting methods. In this method, joints are subjected to not only shear stress but tensile stress through expanded slabs. In this study, we investigated the potential uses of this shear-key to expanded slab. Test specimens had the proposed shear-keys to resist shear stress and adhesive post-installed anchors to resist tensile stress. As a result of the test, the shear-keys also afforded higher shear strength and stiffness under these conditions. Moreover, we constructed the design method for applying the shear-key to expanded slab, and it was obvious that horizontal capacity of expanded slab with the shear-keys could be reasonably estimated by the proposed shear strength formula.