PURPOSE: To enhance the easiness to execute works and the economy by coating auxiliary plates with sealant, fitting the auxiliary plates in a pair of grooves provided in a pair of wall panels, and nailing them fast.

CONSTITUTION: That side face of each auxiliary plate 16 abutting to a structural face bar 3 of an archtectural wall panel 1 is coated with a sealant 15 continued in the plumb direction. The two auxiliary plates 16 are fitted in a pair of fitting grooves 4 in an existing wall panel 1 formed by adhering the structural face bar 3 to both surfaces of a molded insulation foaming 2. The undersurface of other wall panel 1 is fixed to a lower sash member, and the auxiliary plates 16 are fitted in the grooves 4 in the wall panel 1. The sealant 15 is spread approx, uniformly over the whole joining surfaces. Further, nailing fast is made exteriorly, and the two wall panels 1 are joined together through the auxiliary plates 16.