To improve workability of reinforcing H-steel by arranging the fixing part on one end of an arm extended between flanges of H-steel, and arranging the abutting part on the other end.

A stiffener metal fitting 1 is composed of the arm part 2, the bolt fixing part and the abutting part arranged on its both ends to reinforce the flange part by being interposed between a first flange F1 and a second flange F2 of H-steel. Next, stoppers 3c, 4b are arranged on the tip of the bolt fixing part and the abutting part to prevent the metal fitting 1 from entering the web W side. A pair of abutting stands 4a of the abutting part are fixed by being allowed to abut to the flange F2 by allowing the tip abut to the flange F1 by fastening a pari of bolts B1 after allowing the stopper 3c, 4b to abut to the tip parts F1a, F2a of H-steel by inserting the metal fitting 1 between the flanges F1, F2 at reinforcing time. Therefore, the H-steel can be reinforced without requiring welding, a mounting/demounting position can be easily changed, and the H-steel can be reused since it is not damaged at mounting/ demounting time.