PURPOSE: To promote execution efficiency by making weak heat conductive members lie between a column and an exterior wall material, and forming spray foam heat insulating materials on the rear of the exterior wall material from an indoor side directly before the execution of an interior wall material.

CONSTITUTION: An exterior wall material 7 is insulated from a column 3 and studs 4 through columnar grounds 9 formed of weak heat conductive members such as polyethylene, etc., and is mounted. Foamable liquid 13 is sprayed to the exterior wall material 7 in a laminar-shape from a spray gun 12, it is foamed in a site, and a heat insulation air-tight layer 14 is formed of the grounds 9 and spray foam heat insulating materials 8. In addition, an interior wall material 10 is mounted to the column 3, and an air layer 11 is formed between the column 3 and spray foam heat insulating materials 8.