To reduce hammering noise made when a duct is formed by bringing a metal plate to beam coupling.

In beam coupling in which a male beam 4 is provided on a plate 2 with a female beam 6 and a stopper beam 8 provided on a plate 3, and the male beam 4 is fitted to the female beam 6, and the stopper beam 8 is bent for slipping. In the beam coupling, slits 9 are formed in the stopper beam 8 at a small distance to form many tongue pieces 8a, whereby resistance against the bending is substantially prevented from portions other than a blow position. The blown tongue piece 8a is easy to be bent, so that a small blow may be applied, and hence any large noises are prevented from occurring.