PURPOSE: To install a hollow extrusion-molded board to a body by a simple operation without machining at a site.

CONSTITUTION: In a hollow extrusion-molded board 20, in which a surface side board 20a and a rear side board 20b are arranged in parallel while holding a regular interval, holes 21 for bonding bolts are bored previously at the specified positions of the upper and lower end sections of the rear side board 20b respectively. Clips 23, 24 are mounted to the holes 21 for bonding the bolts of the hollow extrusion-molded board 20 through connecting bolts 22 and square nuts 31 at that time. The hollow extrusion-molded boards 20 disposed in parallel up and don are set up to a body side girder 25 through the clips 23, 24 and its own bearer 27.