PURPOSE: To reduce the manufacturing cost and manufacturing man-hours and to keep enough strength even if the upward tension is applied by integrally forming a locking member, a connecting member and a fitting member.

CONSTITUTION: A locking member 15 is installed in a valley part 11B clamped by adjcent parts 11A of a folded plate roof 11. Subsequently both end parts of the respective side plates 15A, 15C are respectively fixed to the root constricted parts of swollen head parts 11C of adjacent crest parts 11A. A fitting member 17 is installed on the top surface of the swollen head part C between the adjacent crest parts 11A, 11A of the roof 11. Thus, the locking member 15, a connecting member 16 and the fitting member 17 are integrally formed so that the need of joining can be eliminated and the manufacturing cost can be reduced.