In 2007, the building standard law of Japan was revised and a specification of column overdesign factor (COF) was added in case of square tube columns. These researches deal with existing non-conformed steel buildings using STKR columns according to being not satisfied with the specification of COF. In this paper, in order to enhance the full-plastic moment of column, reinforcement wrapping mortar around column base is proposed. An experimental verification of reinforced column was conducted to investigate mechanical behavior and confirm the validity of reinforcement design method. As a result, specifications for providing required bending moment to column were clarified.