To achieve lightweight, and obtain reinforcing material by utilizing chip material such as front and rear plates, and make use of waste material.

Cut grooves 11a having their lower ends opened are formed at both the sides of one thin plate material 10a by the use of two sheets of belt-like thin plate materials 10a, 10b having pliability. Also, cut grooves 11b having their upper ends opened are formed at both the sides of other thin plate material 10b in positions corresponding to aforementioned cut grooves 11a. Reinforcing materials 10 are formed by fit-joining both the thin plate materials 10a, 10b with respective cut grooves 11a, 11b. The reinforcing materials 10 are arranged in contact with a cleat 9 deployed within an edge material 5 or a hollow part in its both ends in the longitudinal direction.