Banna-ji is well known as a temple associated with the Ashikaga clan that established Muromachi Shogunate. The first Main Hall was built in 1234, however it was damaged by a stroke of lightning in 1287. The existing Main Hall (Hondo) was reconstructed in 1299. After that, the major renovation was conducted from 1407 to 1432, then it has continued to exist until the present day. During repairs of the Hondo's roof from 2009 to 2010, we carried out research on form and detail of structure and did scientific research. We have almost clarified the change in the Hondo in the Medieval Period using accelerator mass spectrometry (AMS), which judges the age of wood element. In spite of the major renovation in the mid-Muromachi period, the Hondo keeps a late Kamakura-period style. Finally, we can ascertain that the Hondo of Banna-ji has historical evaluation as the oldest medieval Zen style architecture in Eastern Japan.
