This study aims to develop and propose a seismic retrofitting method for unreinforced brick masonry for improving the seismic safety of the 1^st and 3^rd headquarters buildings of Kyushu University. Five brick wall specimens representing parts of the walls that were adjacent to the openings in the headquarters buildings were prepared on a 3/4th scale of the actual structure and they reinforced on either one or sides of their surfaces with reinforced concrete (RC) walls. Horizontal loading experiment was conducted to confirm the reinforcing effect. Results indicated that the maximum load of the specimens (CS01, CS02) reinforced on one side with RC walls was 6.1 to 6.2 times higher than that of the unreinforced specimen. Similarly, the maximum load of the specimens (CD01, CD02) reinforced on both sides with RC walls was 12.6 to 14.2 times higher than that of the unreinforced specimen. The specimen CS02 reached its maximum load at a smaller deformation angle compared to the specimen CS01 because of the effect of twisting. Additionally, the authors also derived and verified a horizontal strength evaluation formula for brick wall specimens reinforced with RC walls. The values calculated using horizontal strength evaluation formulas were close to the experimental values.