PURPOSE: To effectively use an attic space by forming a structure surface of adjacent side face part connected to another roof unit, in a rigid frame structure, and by laying the side surface part of the rigid structure side surface part on a building structure underneath thereof.

CONSTITUTION: A rigid frame structure member 12 is integrally fixed to a roof unit body 11 and the structure surfaces of adjacent side surface parts 11a which are connected to the roof unit body 11, by mans of bolts and nuts. This rigid frame structure is an arched C-channel steel member composed of an upper frame 12a and both side vertical frames 12b. Next, roof units 10 each incorporating the rigid frame structure 12, are connected together so as to form a roof, and then, and are laid on a building unit bearing walls or beams. Accordingly, a small attic room having a large space can be formed.