To form a new exterior wall or a repaired exterior wall, with the improvement of the strength of a building, including a square column as well.

Reinforcement members C are arranged so as to joint slantwise the corner part where a vertically extended sill 4 to form a building frame A, a member such as an eaves girder and a gable girder 5, a horizontally extended main column 1, a square column 2 respectively intersect and are jointed to one another. In addition, corner reinforcement members D are arranged, so as to slantwise cover the square column 2 in a bent state, and a new exterior wall E is formed on the reinforcement members C and D.