In mounting an external wallboard formed from a material containing cement, a plurality of spacers are adhered to a plurality of wall backings each formed of steel or wood, respectively. Each spacer has through holes or concavities. The external wallboard is applied to be fixed by screws or nails to the wall backings. Each screw or nail extends through the corresponding through hole or concavity such that each screw or nail is allowed to be inclined in a space defined by the corresponding through hole or concavity. This construction permits displacement of the external wallboard relative to the wall backings, preventing occurrence of cracks in the external wallboard.