To improve the installation performance of an air-conditioning duct while suppressing a degradation in assembly performance of and an increase in weight of a beam member.

A beam member 100 is provided with an open sectional form and an air-conditioning duct 200 is assembled from the opening, which can reduce the assembly man-hour of the beam member to improve the assembly performance of the beam member. Even when the sectional area of an air passage in the air-conditioning duct 200 is increased the open sectional form of the beam member 100 can prevent an increase in sectional area of the beam member 100 corresponding to the increased size of the air-conditioning duct 200. The beam member 100 can be protected from an unnecessary increase in rigidity and a resultant unnecessary increase in weight.