PURPOSE: To allow a front panel to be fitted in the vertical state between side walls with the upper ends tilting toward the eaves side at the eaves edge.

CONSTITUTION: The side wall 2 of a dormer 3 is formed of a side wall panel 5 having a triangular frame 4 constituted of a lower frame member 11 disposed on an up-beam 20, a front frame member 12 with the upper end tilting toward the eaves side from the vicinity of eaves edge of the lower frame member 11, and an upper frame member 13 connecting the ridge side end of the lower frame member 11 to the upper end of the front frame member 12. A longitudinal backing sash bar 6 vertically extended from the vicinity of the eaves edge of the lower frame member 11 is further formed at the frame 4 of the side wall panel 5, and both side ends of a front panel 9 forming the eaves side front wall 7 of a dormer 3 are fixed to the longitudinal backing sash bars 6.