Shibutani Mukaiyama Kofun is a massive keyhole-shaped burial mound located in Shibutani-cho, Tenri City, Nara Prefecture. It is one of the principal tombs in the Yanagimoto Kofun Group and is particularly notable for its enormous size, measuring approximately 300 meters in total length. Today, the Imperial Household Agency officially designates and manages the site as the mausoleum of Emperor Keikō, traditionally regarded as Japan’s 12th emperor. Its official name is Yamanobe no Michi no Ue no Misさasagi.
The mound stands near the eastern edge of the Nara Basin, at a change in the slope of a ridge extending westward from Mount Ryūō. Its elevated setting and commanding scale make it a prominent feature of the ancient landscape.
Its construction was initially dated to the latter half of the 4th century, but more recent research increasingly supports a date around the middle of the 4th century. Shibutani Mukaiyama Kofun is the largest burial mound in the Yanagimoto Kofun Group and ranks eighth in size among Japan’s ancient burial mounds. It is an important archaeological monument for understanding the development of early Japanese political power.
The mound has the distinctive keyhole shape characteristic of many large Japanese burial mounds, combining a circular rear section with a rectangular front section. The rear section was constructed in four tiers, while the front section has three tiers.
The circular rear section measures approximately 168 meters in diameter and stands about 25 meters high. The rectangular front section is approximately 170 meters wide and 23 meters high. The entire mound extends for roughly 300 meters, reflecting the considerable resources and organizational power required to construct such a monumental tomb during the early Kofun period.
A moat encircles the burial mound. Raised crossings, known as watari-zutsumi, divide it into sections, with six on the rear-section side and four on the front-section side. The front-section moat is believed to have been enlarged during the early modern period for use as an agricultural reservoir. The moat around the rear section, by contrast, is thought to retain more of its original form.
Historical accounts indicate that an Amida Buddha hall and a Kannon hall once stood on the front section before the end of the Edo period. These structures are believed to have altered parts of the original mound, and traces of such changes remain in its present form. An additional platform-like feature has also been identified near the southern foot of the circular rear section.
Archaeological materials associated with Shibutani Mukaiyama Kofun include several types of haniwa, or clay figures placed around burial mounds. These include cylindrical haniwa in ordinary, finned and morning-glory-shaped forms, as well as figurative examples shaped like umbrellas and shields.
These objects provide valuable clues about the mound’s construction period and the rituals associated with elite burials during the Kofun period.
One particularly important object is a stone headrest made of green jasper, reportedly discovered during restoration work in 1864. Weighing approximately 24 kilograms, it features incised lines along its outer edges and sides. The object is designated an Important Cultural Property of Japan and is currently held by the Kansai University Museum in Suita City, Osaka Prefecture.
A triangular-rimmed bronze mirror decorated with deities and animals is also said to have been discovered in Shibutani Village. However, the circumstances of its discovery remain unclear. The mirror is now held by the Kyoto National Museum.
The identity of the person buried in Shibutani Mukaiyama Kofun has not been definitively established. Its official designation has changed over the centuries as scholars and authorities interpreted historical documents and traditions in different ways.
In 1697, during a search for imperial tombs under the Tokugawa shogunate, the mound was considered a possible tomb of either Emperor Sujin or Emperor Keikō. In 1855, it was identified as the mausoleum of Emperor Sujin. Restoration work took place in 1864, when the stone headrest was reportedly discovered. In 1865, shortly before the restoration work was completed, the designation was changed to the mausoleum of Emperor Keikō.
During the Meiji period, the Imperial Household Ministry, the predecessor of today’s Imperial Household Agency, formally designated and began managing the site as Emperor Keikō’s mausoleum.
Since 1971, the Archives and Mausolea Department of the Imperial Household Agency has conducted several investigations related to the site. In 2016, representatives of 15 Japanese archaeological and historical academic societies were also permitted to enter the area for a scholarly investigation.
These investigations have renewed attention to the mound’s archaeological and historical importance. However, because it is managed as an imperial mausoleum, opportunities for direct archaeological examination remain limited.
From an archaeological perspective, Shibutani Mukaiyama Kofun is regarded as one of the major royal burial mounds associated with the early Yamato polity, the political power that helped shape ancient Japan. Although the actual identity of its occupant remains uncertain, its official designation as Emperor Keikō’s mausoleum is based in part on accounts in ancient chronicles and historical records.
The Kojiki, one of Japan’s earliest chronicles, refers to a location called Yamanobe no Michi no Ue. The Nihon Shoki records the name Yamanobe no Michi no Ue no Misさasagi. These references have been interpreted as descriptions of Emperor Keikō’s mausoleum.
The Engishiki, a 10th-century compilation of court regulations and ceremonial procedures, also records the tomb under the name Yamanobe no Michi no Ue no Misさasagi, placing it in Shiki District of Yamato Province. It describes the tomb precinct as measuring two chō from east to west and two chō from north to south, and records the assignment of one guardian household.
Surrounded by the quiet landscape of Tenri City, Shibutani Mukaiyama Kofun stands as a remarkable reminder of the power and sophistication of ancient Japanese society. Its immense scale, distinctive architecture and valuable archaeological finds offer important clues to the development of the early Yamato polity.
Although the identity of the person buried within it remains a subject of scholarly interest, the mound’s association with Emperor Keikō and its place among the great burial monuments of the Kofun period make it an important site for understanding Japan’s ancient history. Continued research and preservation will help protect this exceptional cultural heritage for future generations.