of the reinforcing face plate 50 even if the sizes of the upper and lower transverse frame members 4 5 and the pair of left and right vertical frame members 6 6 are reduced the frame 7 is not deformed by the weight of the stone plate 8 further since the stone plate 8 rests on the stone resting piece 27 of the lower transverse frame member 5 no high stress is applied on the adhesively bonded portions and therefore it is possible to prevent the stone plate 8 from detaching from the frame 7 further as shown in fig2 the intermediate transverse frame members 9 in the embodiment shown is formed in an approximately rectangular hollow shape by an indoor side vertical plate 40 an outdoor side vertical plate 41 an upper plate 42 and a lower plate 43 the upper and lower plates 42 and 43 have an upward piece 44a and a downward piece 44b formed integrally therewith at their respective inner ends further the upper and lower plates 42 and 43 have a rising piece 45 and a hanging down piece 46 formed integrally therewith at their respective outer ends so that they may be located flush with the outside surface of the outdoor side longitudinal plate 41 further the outdoor side vertical plate 41 has.