To improve fire resistance without filling a joint part with a refractory material otherwise by filling an outer wall joint gap with a refractory resilient backup material and tightly adhering a seal material on the backup material from the outside to be filled.

A seal backup material 18 comprising a material provided with fire resistance is inserted in a lateral joint part between upper and lower wall panels 8, 8, filling is performed into the gap between siding materials 1, 1 up to a position abutted to a lower lateral frame from the outside, and a seal material 19 is tightly adhered on the outside of the refractory backup material 18 to be filled. The joint part is a double structure of the backup material 18 comprising a refractory material and the normal seal material 19, both are adhered, at the time of a fire, in the case where a flame invades from the outside and transfers to the seal material 19, the seal material 19 is carbonized, but it does not burn out, and fire resistance property is improved together with the backup material 18. Thus, the refractory material need not be filled otherwise, and working man-hours can be reduced.