PURPOSE: To increase the internal pressure of a space section, and to prevent wind load and the intrusion of rainwater by forming an air chamber having a plurality of partitions by a partition material in the space section when the space section is shaped between a main external wall and a sheath material.

CONSTITUTION: A space section 43 is shaped between the main external wall 35 and sub-external wall 31 of a building 33. Air on the surface side and the rear side is communicated through communication paths 45 among a plurality of sheath materials 41 of the sub-external wall 31. The space section 43 is partitioned by partition materials 47 installed to the upper sections of the sheath materials 41, thus forming a plurality of air chambers 49. Accordingly, the internal pressure of the air sections of the rears of the sheath materials 41 is elevated sufficiently in response to the increase of wind pressure working to the sheath materials 41, thus largely reducing wind load and intrusion into the space section of rainwater.