Odachi inscribed Teiji 5th Year, Senjuin Nagayoshi

Senjuin Nagayoshi was a swordsmith of the Yamato Senjuin school, a major tradition centered around the Senjuin area of Yamato Province (modern Nara), which supplied swords to the warrior monks and samurai of the Nara region from the late Heian period into the Muromachi era. The Yamato tradition (Yamato-den) is characterized by bo-hi grooves, Sanskrit carvings, and a distinctive forging style. Nagayoshi was a leading smith of the Nanbokucho era who excelled particularly in large swords.

Oyamazumi Shrine on Omishima Island in Ehime Prefecture is one of Japan's oldest shrines, dedicated to the mountain and sea deity Oyamazumi-no-kami and revered as the supreme guardian of warriors. Over the centuries, countless suits of armor and swords were dedicated here; the shrine's treasury now holds eight National Treasures and 469 Important Cultural Properties in arms and armor — approximately 80% of all nationally designated armor in Japan, earning it the title "Arsenal of the Nation."

This odachi (great sword), inscribed with the date Teiji 5 (1366) and the maker's name Senjuin Nagayoshi, is a masterwork of the Nanbokucho great-sword style. With a blade length of 135.7 cm and curvature of 4.8 cm, it represents the technical achievement of forging a blade of extraordinary length without defect. It was traditionally said to have been dedicated to the shrine by Emperor Gomurakami (r. 1339–1368). Designated an Important Cultural Property in 1911 and a National Treasure on March 31, 1953, it remains at the Oyamazumi Shrine treasury museum.

The blade features flowing itame ji-hada, the characteristic Yamato bo-hi groove, and a hamon of ko-midare with gunome patterns. As the only dated and signed (1366) great sword by Nagayoshi, it serves as a benchmark for dating and attributing Yamato Senjuin-school production.