PURPOSE: To prevent a large mass of snow with which an extension extending outward from an inclined roof is covered, from dropping, so as to enhance the safety.

CONSTITUTION: A dormer 2 having an outer surface B which is positioned so as to face downward in the direction of an inclined roof R of a building H is formed on an extension A extending outward from the inclined roof R. Common rafters 6... are supported by a front face beam 5 which is attached to parallel cantilever beams 4, 4 extending outward from a beam 3 laid along the eaves R1 of the inclined roof R, and support columns 9, 9 arranged on opposite sides of the outer surface B of the dormer 2 are planted on the front beam 5.