Moisture damage caused by mold and rust occurred in the storage of the building which had been thought to be less affected by the external environment. The building examined is located on a mountaintop, wherein the high humidity of the outside air tends to persist throughout the year, which was identified as the cause of the damage. The improvement measures were to reduce the relative humidity below 65% at natural room temperature and to install an energy-efficient system. A heat pump desiccant humidity control outdoor air unit was installed, and a positive pressure system was used because the building was considered to be airtight low. As a result, significant environmental improvements have been achieved, but the underlying mechanism has not been clarified and no design method has been established to apply to other storages. The purpose of this paper is to clarify the formation mechanism of the temperature and humidity environment of the storage and exhibition case, and to clarify the air flow from the measurement of the temperature and humidity and carbon dioxide concentration in the space before and after the operation of the facility.