To cover and reinforce an existing column structure firmly with a steel sheet simply and economically.

Face forms A1-A4 formed in response to each face plate d1-d4 of a reinforcing frame D covered and arranged around an existing column B and made of a steel sheet are combined in a series at the end sections of the horizontal batter materials 1, 1 of the face forms. These face forms A1-A4 are disposed on each face plate d1-d4 and the outer end sections of the face forms A2, A3 placed at outer ends are united mutually in a separable manner by using clamping bolts 7. The horizontal batter materials 1, 1 are mounted so that the nodes of the horizontal batter materials 1, 1 are placed on sides inner than the ends of the face plates d1-d4 of the reinforcing frame D by a considerable distance L.