Ōdachi by Senjuin Nagayoshi: A National Treasure Great Sword at Oyamazumi Shrine

On the island of Omishima in the Seto Inland Sea stands Oyamazumi Shrine, one of Japan's most revered Shinto sanctuaries and a pilgrimage site for warriors since ancient times. Among its extraordinary collection of arms and armor — said to hold roughly 80% of Japan's nationally designated samurai weaponry — one artifact stands apart: the ōdachi (great sword) inscribed "Jōji 5th Year, Hinoe-Uma, Senjuin Nagayoshi." Designated a National Treasure in 1953, this massive blade is regarded as the finest surviving example of the nodachi-style great swords that were in vogue during the tumultuous Nanbokuchō period (1336–1392).

What Is an Ōdachi?

An ōdachi — also known as a nodachi (field sword) — is a category of Japanese sword distinguished by its extraordinary length. While a standard tachi typically has a blade of 70–80 cm, an ōdachi exceeds that range dramatically. This particular example measures an impressive 135.7 cm in blade length alone, with a tang (nakago) adding another 37.6 cm, bringing the total to over 173 cm. During the Nanbokuchō period, civil war between the Northern and Southern Imperial Courts drove demand for increasingly large and powerful weapons. The great sword became a symbol of martial prowess, intended for use on open battlefields where reach and cutting power were decisive advantages.

The Senjuin School: Swordsmiths of Nara

The blade was forged by Nagayoshi, a master of the Senjuin school (千手院派), one of the celebrated "Five Schools of Yamato" (Yamato Goha) based in Nara. The Senjuin school is considered the oldest of these five lineages, tracing its origins to the late Heian period when the founder Yukinobu served as a resident smith at Senjuin, a sub-temple of the great Tōdai-ji. The school's smiths were closely associated with the warrior monks (sōhei) of Tōdai-ji, forging weapons for their military campaigns.

What makes this sword particularly significant is that signed works by the Senjuin school are exceptionally rare. In their earlier periods, the smiths considered it improper to inscribe their names on weapons destined for temple use, and many blades were treated as consumable tools of war. This ōdachi, however, bears a detailed inscription on the reverse of the tang reading "Jōji gonnen hinoe-uma Senjuin Nagayoshi" (貞治五年丙午千手院長吉), firmly dating it to 1366 and attributing it to the smith Nagayoshi — making it an invaluable document of Japanese sword history.

Why Was It Designated a National Treasure?

The sword was first recognized as an Important Cultural Property (then called a National Treasure under the old system) in 1911, and elevated to its current National Treasure status on March 31, 1953, under the revised Cultural Properties Protection Act. Several qualities contribute to its supreme importance:

• It is the representative masterwork of the Nanbokuchō-era nodachi tradition, demonstrating that such an enormously long blade could be forged without technical compromise.
• The steel's grain pattern (jihada) shows a flowing itame mixed with masame, with fine nie particles (ji-nie) and patches of ji-zuma — a sophisticated metallurgical achievement at this scale.
• The hamon (temper line) features a lively pattern of ko-gunome mixed with ko-midare, with plentiful ashi and ha working, and a tight, nie-based nioiguchi — hallmarks of a highly skilled Yamato tradition smith.
• The tang (nakago) is in its original, unshortened (ubu) condition, preserving the full inscription — a rarity among swords of this age.
• The blade and its metallurgical structure remain remarkably healthy despite being over 650 years old.

The Legend of Emperor Go-Murakami's Offering

According to shrine tradition, this great sword was offered to Oyamazumi Shrine by Emperor Go-Murakami (1328–1368), the second emperor of the Southern Court during the Nanbokuchō civil war. Go-Murakami's reign was marked by constant military struggle against the Northern Court backed by the Ashikaga shogunate. As Oyamazumi Shrine was revered as a deity of warfare and safe passage, it was a natural destination for imperial prayers and votive offerings. The date of the inscription — 1366 — falls within the final years of Go-Murakami's reign, lending historical plausibility to this tradition.

Physical Characteristics

The sword is constructed in the shinogi-zukuri style with an iori-mune (gabled ridge). Its proportions are striking: a blade width (motohaba) of 4.1 cm at the base tapering to 3.0 cm at the tip (sakihaba), with a pronounced 4.8 cm curvature and a large kissaki (point) measuring 6.8 cm. Both sides bear deeply carved bō-hi (grooves) paired with companion grooves (tsure-hi), ending in a rounded maru-dome at the base. These grooves serve both an aesthetic and a functional purpose, lightening the blade while producing the characteristic whistling sound when swung — a feature especially valued in battlefield swords of this era.

Oyamazumi Shrine: Japan's Greatest Samurai Treasury

Oyamazumi Shrine (大山祇神社) on Omishima Island is the head shrine of all Oyamazumi and Mishima shrines across Japan, numbering over 10,000. Dedicated to Ōyamatsumi-no-Ōkami — a deity of mountains, seas, and warfare — the shrine has attracted warrior pilgrims since at least the 6th century. Samurai would pray here for victory before battle and, upon returning victorious, offer their arms and armor in gratitude.

This tradition has resulted in one of the most remarkable collections of historic weaponry anywhere in the world. The shrine's Treasure Hall (Hōmotsukan), divided into the National Treasure Hall (Kokuhōkan) and the Shiyōden annex, houses eight National Treasures and dozens of Important Cultural Properties. Among these are suits of armor associated with Minamoto no Yoritomo, Minamoto no Yoshitsune, and the legendary female warrior Tsuruhime, as well as the even larger ōdachi attributed to Ōmori Hikoshichi (blade length 180 cm). The shrine grounds are also home to magnificent camphor trees, some estimated to be 2,600 to 3,000 years old and designated as a national natural monument.

Visiting the Ōdachi

The ōdachi by Senjuin Nagayoshi is displayed in the Treasure Hall within the Oyamazumi Shrine precincts. To see it, visitors purchase a combined ticket that also grants access to the Omishima Maritime Museum. The Treasure Hall's first floor presents a striking array of unsheathed blades — many showing the nicks and wear of actual combat — while the upper floors display armor and other artifacts. The dimly lit display rooms create a solemn, almost reverent atmosphere that heightens the experience of encountering these centuries-old treasures.

Note that photography is not permitted inside the museum buildings. English labels provide basic identification for each exhibit, though detailed explanations are primarily in Japanese. A visit of about one hour is recommended for the combined museum facilities.

Surrounding Attractions

Omishima Island offers much more beyond the shrine. The island is a popular stop along the Shimanami Kaido cycling route connecting Onomichi (Hiroshima Prefecture) to Imabari (Ehime Prefecture) via a series of spectacular bridges. Nearby attractions include the Toyo Ito Museum of Architecture, the Omishima Museum of Art, and the scenic coastal road that winds past citrus orchards with panoramic views of the Seto Inland Sea. The island also features the Tatara Shimanami Park road station, local seafood restaurants, and the Maré Glassia Omishima marine spa facility.