The West Cocoon Warehouse, located within the former Tomioka Silk Mill, a UNESCO World Heritage Site, was constructed when the mill commenced operations in 1872 and was designated as a National Treasure of Japan in 2014. The West Cocoon Warehouse was a large-scale facility essential for storing cocoons to ensure a stable production of raw silk throughout the year. It features a distinctive form with a length exceeding 100 m on its long side and is a timber-masonry composite structure with two storys. The conservation work on this building was completed by May 2020, and it is now being used as a multi-purpose hall that can be used not only for tours but also by citizens and others. In this conservation work, a solution was sought for the complex issues of preservation and utilization as well as seismic reinforcement. The authors performed vibration measurements before the conservation work and identified that the predominant vibration mode was a bow-shaped mode with a relatively long period of approximately 1.14 s in the out-of-plane direction of the wall surface in the West Cocoon Warehouse, which has a length of over 100 m. Furthermore, after the conservation work was completed, vibration measurements were taken again to clarify the basic vibration characteristics of the timber-masonry composite building that had been repaired and reinforced to improve its seismic performance.