PURPOSE: To magnify a dormer, by forming the dormer by use of the frontal structure, the ridge beam, and sashes with a framework method and mounting the end of eaves of ridge beam on a ridge beam support and tilting the ridge beam by a specified angle.

CONSTITUTION: A dormer A is formed by a front wall 10 and side walls 12 erected so as to surround the eaves side of an opening 1 provided on a roof, and a roof face 11. A U-shaped frontal structure 2 is erected at the front end of the eaves side of the opening 1 and a ridge beam 6 is erected between the ridge beam support 5 projected from a ridge beam support member 4 and the rear end of the ridge side of the opening 1. Next, sashes 9 forming the ceiling underlayer 7 and the wall underlayer 8 along the longitudinal direction of the ridge beam 6 are arranged so as to cross at right angles with the ridge beam 6. The ridge beam support 5 of the ridge beam support member 4 of the ridge beam 6 is inclined. In this way, the load handling can be kept well by use of the framework method.