PURPOSE: To prevent dew condensation in a vent space formed in a partition and to enable to perform automatic ventilation of the interior of a chamber, by a method wherein an inlet hole is formed in a partition wall a given distance away from an outlet hole, and a ventilating fan, adapted to exhaust the air in a vent space to the outside of a building, is located in the outlet hole.

CONSTITUTION: An outlet hole 11, adapted to communicate the outside 10 of a building 1 with a vent space 9 and exhaust the air in the vent space 9 into the open air, is formed in about the portion, where a partition wall 3 is partitioned, of an outer wall 2, and a ventilating fan 12 is disposed therein. The ventilating fan 12 is actuated when humidity in the vent space 9 exceeds a previously determined value, and is switched ON and OFF by means of a humidity sensor 13 situated in the vent space 9. An inlet hole 14, through which the vent space 9 and an interior 4a of a chamber are intercommunicated, is formed in the partition wall 3. The inlet hole is formed in a position located a given distance so away from the outlet hole 11 as the allow sufficient ventilation of the interior of the vent space 9.