The treasure house of Horyu-ji Temple is a non-air-conditioned storage facility that houses wall paintings in the main hall of Horyu-ji Temple, which was damaged by fire in 1949. Currently, mold has been observed on the wall paintings stored in the central room of the treasure house. To safely preserve the wall paintings, it is necessary to prevent mold growth. The final goal of this study is to propose environmental management methods for the safe preservation and public display of murals by reducing the risk of mold growth in the treasure house of Horyu-ji Temple. To clarify the current temperature and humidity conditions inside the storage room, field measurements of the temperature and humidity were conducted in the storage rooms. In addition, to clarify the mechanism of temperature and humidity environment formation in the central room, the models of the central room were developed and CFD transient simulations were performed. The results of measurements in the storage room revealed that the relative humidity in the central room exceeds 60% throughout most of the year. Furthermore, the temperature in the lower part of the room, especially behind the wall paintings, is more than 2 ℃ lower than in the upper part of the room, resulting in a high humidity environment exceeding 70% during the summer. The CFD analysis revealed the cause of the lower part of the room being cooler than the upper part, and part of the mechanism by which the space behind the wall paintings is cooler than the space in front of them. This study provides a method to reproduce the problematic distribution of temperature in the non-air-conditioned storage environment of cultural properties and clarify the formation mechanism of temperature distribution.