Generally, in reinforced concrete buildings, joints have been planned as a rigid state due to the characteristics of their construction. In such a building, the stress of the first story column becomes nonuniform since the bending stress at the time of the earthquake is concentrated on the rigid joints of the foundation structure such as the column base at the first story. This paper proposed making the column base at the first story shape semi-rigid to reduce the stress concentrating on the column base at the first story. In order to realize semi-rigid column base, we performed structural experiments and FEM analysis on the column base with cross section reduced portion. As a result, it was concluded that the best structure shape is one with the cross section reduced portion and the taper at the column base as the semi-rigid column base with bending rigidity reduction and structural performance stability.