PURPOSE: To mount roof panels with a high degree of accuracy and easily even if there are distortions such as warp, etc., in them.

CONSTITUTION: Struts 3 are fabricated by fitting one of an upper member 3A and a lower member 3B, both made of shape steel, into another. A bolt-hole capable through which one side piece of the upper member 3A is fixed to a frame of one roof panel 2 is bored in the upper part thereof, the lower part is so formed in a smaller diameter that it can be fitted to the lower member 3B, and both upper and lower members 3A and 3B can be bolted. According to the constitution, even if there are distortions such as warp, etc., in the roof panels, the panels are mounted with a high degree of accuracy and easily.