To provide an assembly tool and an assembling method for easier and faster scaffold assembly to help safe roofing work on a slant roof surface.

This assembly tool consists of a fall prevention supporting post 3 erected in forwardly slant position at angle S substantially equivalent to of the slant roof, on the rear side surface of a base frame 2 that has a projected hook 1 at the front edge, a scaffold-supporting arm 4 put up from the front edge of the base frame 2 to cross at almost right angles with the core supporting post. This assembly tool for fall-preventing scaffold, for roofing work is so designed that the support 3 is set in vertical position and the arm 4 is set in horizontal position when it is provided along the slant of the roof. A plurality of the assembly tools are arranged on the slant roof and detachably connected to each other. Scaffold boards are put on the arm 4 for supporting the scaffold to assemble it.