In order to understand the characteristics of water vapor transport and corrosion mechanisms of bolt cap made by polyvinyl chloride resin, temperature and humidity inside the bolt cap exposed to the atmospheric environment was monitored. As a result of estimating the amount of water vapor transportation from the monitored data, it was confirmed that water vapor transportation was observed to the bolt cap. On the other hand, corrosion of steel installed inside the bolt caps was suppressed, indicating that the bolt cap has a high anti-corrosion effect. The reason for this result is thought to be that the resin material of the bolt caps has low oxygen permeability, limiting the supply of oxygen necessary for steel corrosion,
