Andonyama Kofun, also known as Andon-yama Kofun, is an ancient keyhole-shaped burial mound located in Yanagimoto-cho, Tenri City, Nara Prefecture. Believed to have been constructed during the early Kofun period in the first half of the 4th century, it forms part of the Yanagimoto Kofun Group, a concentration of large burial mounds in the region. Among these ancient monuments, Andonyama Kofun is particularly notable for its impressive scale and its association with the formation of early Japanese political authority.
The Imperial Household Agency officially designates Andonyama Kofun as the mausoleum of Emperor Sujin, traditionally regarded as Japan’s 10th emperor. Its imperial designation is Yamanobe no Magari no Oka no Misさasagi, commonly rendered in English as the Imperial Mausoleum of Emperor Sujin. The site is managed as an imperial tomb, and access to the burial area is restricted.
The identity of the person actually buried here has not been conclusively established. Nevertheless, the mound has long been associated with Emperor Sujin through historical traditions and its designation in official records.
Andonyama Kofun is estimated to date from the early 4th century, during the early Kofun period. It has the distinctive keyhole shape characteristic of many major Japanese burial mounds, combining a circular rear section with a rectangular front section. The front section faces northwest, and the mound was constructed in three tiers.
The mound measures approximately 242 meters in total length, making it one of the largest burial mounds in Japan. Its scale reflects the considerable political and social power of the elite who commissioned such monumental tombs during the early centuries of the Japanese state.
The circular rear section measures approximately 158 meters in diameter and reaches a height of 31 meters. The rectangular front section is about 100 meters wide and 13.6 meters high.
A shield-shaped moat surrounds the mound, extending approximately 360 meters in total length. Three raised crossings, known as watari-zutsumi, interrupt the moat. During the late Edo period, part of the moat was enlarged for use as an agricultural reservoir, illustrating how the landscape around the ancient monument was adapted to meet later community needs.
Several types of archaeological material have been associated with Andonyama Kofun. One particularly intriguing find is a bronze plate reportedly discovered during tomb restoration work in the Edo period. It was rectangular, measuring approximately 70 centimeters along its longer side and 53.8 centimeters along its shorter side.
Descriptions indicate that the plate bore patterns resembling the circular designs of an inner-flower-pattern mirror, along with a grid-like motif divided into four sections. The original object is now missing, but a rubbing of its surface survives, preserving a record of this unusual discovery.
Investigations conducted by the Archives and Mausolea Department of the Imperial Household Agency also recovered cylindrical haniwa clay figures, Haji pottery and Sue pottery. These materials display characteristics associated with the early Kofun period and provide valuable evidence for understanding the mound’s date and the funerary practices of the time.
Within the Yanagimoto Kofun Group, Andonyama Kofun is second in size only to Shibutani Mukaiyama Kofun, which measures approximately 300 meters in length. Its monumental dimensions have led researchers to regard it as one of the major royal burial mounds associated with the emergence of the early Yamato polity, a political power that played a central role in the formation of ancient Japan.
Together with other important sites, including Hashihaka Kofun in Sakurai City, Andonyama Kofun offers clues to the development of the Yamato court and the gradual emergence of centralized political authority. Although many questions about its construction and occupant remain unresolved, the site is an important reference point for archaeological and historical studies of early Japan.
The mound’s official identification has changed over time, reflecting evolving interpretations of historical records and imperial genealogy.
Between 1797 and 1801, during the Kansei and Kyowa eras, the scholar Gamō Kunpei identified the site as the mausoleum of Emperor Keikō in his work Sanryōshi, a study of imperial tombs. In 1855, it was officially designated as the mausoleum of Emperor Keikō. In 1865, the designation was changed to Emperor Sujin.
During the Meiji period, the Imperial Household Ministry, the predecessor of today’s Imperial Household Agency, began managing the site as Emperor Sujin’s mausoleum. Archaeological investigations by the agency’s Archives and Mausolea Department took place in 1974 and 1975. In 2017 and 2020, researchers from several archaeological and historical academic societies also conducted investigations involving entry into the site.
These developments demonstrate the continuing scholarly interest in Andonyama Kofun and how its interpretation has changed across different historical periods.
Andonyama Kofun is associated with four satellite burial mounds, known in Japanese as baichō. Such mounds were constructed near a principal tomb and may have served related funerary purposes.
The largest of these is Ando-yama Kofun, located north of the imperial tomb’s worship area. It is itself a keyhole-shaped burial mound measuring approximately 120 meters in length. Three other associated mounds, known as the I-go, Ro-go and Ha-go mounds, are situated separately from the main burial mound.
Nearby Chōgaku-ji Temple is associated with an intriguing object known as the Miroku Stone Coffin Buddha, a Buddhist image carved from a stone coffin. One theory suggests that the stone used for the image was originally a ceiling stone from the burial chamber of Andonyama Kofun.
Although this connection remains a theory, it raises interesting questions about how materials from ancient burial sites may have been reused in later religious settings. It also illustrates the complex relationship between archaeological remains and the region’s subsequent religious and cultural history.
The traditional association between Andonyama Kofun and Emperor Sujin is supported by references in the Kojiki and the Nihon Shoki, two foundational chronicles of ancient Japan. The mound is also connected with the imperial tomb recorded in the Engishiki, a compilation of court regulations and ceremonial procedures completed in the 10th century. The tomb is referred to there as Yamanobe no Michi no Ue no Misさasagi, and the text records provisions for a guardian household assigned to the site.
These historical references help explain why Andonyama Kofun has long been regarded as an important imperial tomb. Beyond its impressive physical dimensions, it represents an important point of reference for studying ancient royal authority, burial traditions and the development of Japan’s early state.
With its monumental scale, archaeological finds and longstanding association with Emperor Sujin, Andonyama Kofun is one of the most significant ancient sites in Tenri City. It forms part of a wider landscape of burial mounds that sheds light on the rise of the Yamato polity and the early history of Japan.
Although the imperial tomb itself is not open for unrestricted access, visitors exploring the Tenri and Yanagimoto area can appreciate the remarkable historical landscape surrounding the mound and discover the enduring legacy of Japan’s ancient burial traditions.