To easily mold a plate member molded in a belt-shape by bending it at a specific position and, at the same time, to meet with dimensional variations in a beam by only varying a bending position so as to adjust the dimensional variation when the beam having a different size is mounted.

A belt-shaped plate body 16 having specific width cut in a long trapezoidal shape is bent at right angles to right and left in the same direction in a state to ensure width dimension approximately equivalent to a beam in the longitudinal central part to obtain a mounting section 14 held between both extension sections 16c. The extension sections 16c are bent at right angles to the same direction as the bending direction of the mounting section 14 by the specific width parallel with opening end edges to obtain interlocking sections 12. The extension sections 16c are bent at right angles with a diagonal connecting a point intersecting a bending site of the mounting section 14 with a trapezoidal short side to a point intersecting the bending site of the mounting section 14 with a trapezoidal long side to obtain collar sections 17. By the constitution, a joint device 10 for tilting the mounting section 14 of the beam to the locking sections 12 is obtained.