To solve a problem of having small resistance force in an out-of-plane direction in a reinforcing method with a steel plate embedded, in an existing flat column.

A reinforcing steel plate 7 is fixed with a space 11 parallel to an outer surface 10 of the existing column 1, and a vertical reinforcing attached member 13 is joined to the reinforcing steel plate 7. Hoops 15, 15a are arranged surrounding the reinforcing steel plate 7 and the reinforcing attached member 13, and a reinforcing column 3 is formed of concrete placed within a form surrounding the space 11 and hoops 15, 15a. With the reinforcing column 3 thus integrally formed at one face of the existing column 1, the buckling resistance force of the reinforcing steel plate 7 is increased by the reinforcing attached member 13 joined to the reinforcing steel plate 7, to improve resistance force in the out-of-plane direction. This reinforcing structure is adopted particularly to the flat existing column 1 to improve resistance force while reducing a projecting quantity in the out-of-plane direction with the flat reinforcing column 3.