PURPOSE: To remarkably shorten the construction period and to enhance the workability and a safety by enabling the assembly of a roof with steel structure trusses on the ground simultaneously with the construction of a lower structural body in a stable condition in which the horizontal thrust load of a steel truss is reduced.

CONSTITUTION: Crosswise movable temporary masts 18 are supported on a crosswise shift rail 12 so as to assemble steel trusses 13. Further, assembled trusses 13 are crosswise shifted by a predetermined distance along the rail 12 together with the masts 18. Then, the steel trusses 13 are joined together. With the repetitions of the above-mentioned successive steps, a steel structure roof is assembled on the ground, and then, is lifted up together with the rail 12 and the masts 18, and then the rail 12 is crosswise moved, so as to set only the steel structure roof on a lower structural body.