PURPOSE: To eliminate the thermal extension or contraction difference of a heat-insulating mullion and a metallic fitting tool and dewing or the like, by forming an absorbing part of extended or contracted differences and a heat- insulating part, in the matallic fitting tool.

CONSTITUTION: A metallic backing material 32 is fixed to a mullion 10a with screws 34, within a recessed groove provided along the longitudinal axis of the wooden mullion 10a. Two rows of slit extending to the longitudinal direction of the mullion 10a are formed on the backing material 32. The bolt heads of fitting bolts 36a, 36b are inserted in these slits respectively. An aluminum member 38 constituting the metallic fitting tool 30 is fixed to the front end of the bolts 36a, 36b with nuts. Since the bolts 36a, 36b are movable along the slits, the thermal extension or contraction differences between the fitting tool 30 and the mullion 10a can be absorbed. Belt-form heat-insulating materials 48a, 48b are set between the member 38 and the external end member 44 to insulate the heat between the members 38, 44 and cut off the heat transfer between the indoor and outdoor sides through the fitting tool 30 and further, prevent dew drops arisen on the fitting tool 30 and others.