This paper proposes a seismic retrofitting technique for existing R/C buildings by attaching steel bracing frames to the columns projected from the exterior walls of buildings. Two types of the experimental parameters were focussed in this paper. One is the joint technique between the retrofitting steel columns (beam) and the R/C column (beam). The other is whether or not to anchor the base of the steel columns to the foundations of R/C frames. The cyclic lateral loads were applied to the model specimens, which were about 1/3 the size of R/C frames in school buildings under a constant axial load in the experiments. The experiments and the analyses leaded the following conclusion. The full lateral load carrying capacity was developed in all the specimens regardless of the joint techniques between the steel frames and the R/C frames if the bases of the steel frames were anchored to the foundations of R/C frames. However, the R/C columns failed in shear if they were not anchored.