Koizumi Otsuka Kofun is an ancient keyhole-shaped burial mound located in Koizumi-cho, Yamatokoriyama City, Nara Prefecture. It stands on a ridge at the southeastern edge of the Yata Hills, in the northwestern part of the Nara Basin. Designated a Nara Prefectural Historic Site on March 19, 1999, the tomb is an important archaeological landmark that offers insights into the power, burial customs, and craftsmanship of Japan’s early ruling elite.
The mound is particularly notable for its stone burial chamber and the large number of bronze mirrors discovered among its grave goods. Together, these features provide valuable evidence of the status and cultural connections of powerful local leaders during the early Kofun period.
The tomb was constructed using the raised contours of a natural ridge. It measures approximately 88 meters in total length. The circular rear section is about 50 meters in diameter and 7 meters high, while the rectangular front section is approximately 40 meters wide and 2 meters high.
These measurements are estimates, as parts of the mound’s lower slopes were cut away during later land development. Despite these alterations, the overall form remains an important clue to the original design of the tomb.
Koizumi Otsuka Kofun is believed to have been built between the late 3rd and early 4th centuries, during the early Kofun period. It is one of the relatively rare early burial mounds in the northwestern Nara Basin associated with a stone-lined vertical burial chamber.
The discovery of at least seven bronze mirrors makes the site especially significant. Mirrors were prestigious objects in ancient Japan and could symbolize authority, status, and connections with wider political and cultural networks. Their presence suggests that the person buried here belonged to an influential ruling family or local leadership group.
The tomb was already known during the Edo period. It appears in the illustrated geographical guide Yamato Meisho Zue as the tomb of Tomi no Ason Akatsu, a figure associated with early Japanese historical traditions.
Archaeological investigations were conducted in 1962 and 1996. The findings helped clarify the mound’s structure and the nature of its burial goods. In recognition of its historical and archaeological value, the site was designated a Nara Prefectural Historic Site in 1999.
The rectangular front section extends toward the west. No paving stones known as fukiishi or clay figures known as haniwa have been confirmed on the mound’s exterior. The lower slopes were altered during later residential development, leaving traces of this transformation around the site.
The circular rear section was constructed in two tiers, with a vertical stone burial chamber positioned near its center. As with the front section, no confirmed remains of paving stones or haniwa have been identified on the exterior or lower slopes. The relatively simple appearance of the mound distinguishes it from some other ancient Japanese tombs decorated with extensive stone coverings or rows of clay figures.
The burial chamber runs north to south, perpendicular to the mound’s main axis. It measures approximately 5.5 meters in length, 0.7–1.1 meters in width, and 1.3–1.5 meters in height.
Its construction combines basalt from Shibayama in present-day Osaka Prefecture with gneissose granite from the Yata Hills. The stones were arranged in a distinctive roof-like structure, with the sides leaning inward toward the top. This construction method demonstrates the specialized stoneworking skills available to the community that built the tomb.
A U-shaped clay bed remains on the floor of the burial chamber, suggesting that a split-log wooden coffin may once have rested there. The heavy stone slabs were arranged so that the upper courses projected inward, creating a narrowing interior.
Comparable construction techniques have been identified at Tenjinyama Kofun in Tenri City and Mesuriyama Kofun in Sakurai City. These similarities provide useful evidence for studying the development and spread of burial architecture in ancient Yamato.
Excavations uncovered at least seven bronze mirrors featuring a variety of decorative designs. These include mirrors with floral patterns, animal motifs, beast-head designs, and elaborate compositions featuring divine and mythical creatures.
The number and variety of mirrors are among the most remarkable aspects of the tomb. Such objects were valuable possessions in ancient Japan, and their inclusion in a burial can help archaeologists investigate the status of the deceased and the cultural influences reaching the Nara Basin.
Other finds include one iron sword, one iron axe, two small knives, one chisel, and one woodworking tool known as a yariganna. Fragments of jar-shaped pottery were also recovered.
These objects shed light on burial practices and the customs of placing weapons and tools in tombs. They also provide clues about the technologies and material culture of communities living during the early centuries of Japan’s Kofun period.
Following its appearance in Edo-period geographical literature, the tomb became the subject of modern archaeological research. In 1962, the Nara Prefectural Board of Education conducted an excavation in connection with the development of prefectural housing. The results were published in 1966.
In 1996, the Nara Prefectural Archaeological Institute of Kashihara carried out a further investigation of the circular rear section. A report published in 1997 helped clarify the mound’s structure and the artifacts recovered from it.
Although the surrounding area has continued to undergo development, sections of the archaeological site are protected by restricted-access zones. Information boards at selected locations help visitors understand the tomb’s historical importance.
Visitors should remain on permitted paths and follow all signs. Entering restricted areas or climbing onto protected sections of the mound can damage the archaeological remains.
Rokudoyama Kofun lies immediately east of Koizumi Otsuka Kofun. It is another substantial burial mound, with a shape resembling a scallop-shell or narrow-fronted keyhole tomb. An emergency archaeological investigation of its lower slopes was conducted in 1991, revealing important details about its size and structure.
The mound is approximately 100 meters long. Its circular rear section measures about 75 meters in diameter and 14 meters in height, while the front section is around 50 meters wide and 6 meters high. A horseshoe-shaped terrace approximately 50 meters wide surrounds the mound.
Rokudoyama Kofun is thought to date from the late 5th to early 6th century, making it several generations later than Koizumi Otsuka Kofun. It is considered one of the largest burial mounds in the northwestern Nara Basin.
Excavations have identified cylindrical haniwa and other figurative clay objects on the mound’s exterior. These figures may originally have been arranged along the summit or around the junction between the front and rear sections. The surrounding terrace may also have served various purposes in later periods, including agricultural land development and ritual activities.
Together, Koizumi Otsuka Kofun and Rokudoyama Kofun illustrate changes in the burial traditions of powerful local leaders between the early and later Kofun periods. Differences in their shapes, construction methods, and grave goods offer clues to changing political structures, trade networks, and technological exchange.
The burial facilities at Rokudoyama Kofun have not yet been fully investigated, and its grave goods remain largely unknown. Further research and careful preservation could reveal more about the region’s ruling elite during the later Kofun period.
Artifacts from Koizumi Otsuka Kofun are held and displayed at the Museum of the Archaeological Institute of Kashihara in Kashihara City. Its collection includes objects such as mirrors, weapons, and pottery that help explain the burial customs and material culture of ancient Japan.
A visit to the museum can complement a walk around the tomb by allowing visitors to learn more about the objects discovered during archaeological investigations and the historical context in which they were used.
By train: From Tsutsui Station on the Kintetsu Kashihara Line, the tomb is approximately a 15-minute walk.
By car: From Koriyama Interchange on the Nishi-Meihan Expressway, follow National Route 24 and the local roads toward the site. There is no free parking lot dedicated to visitors.
Some areas around the tomb are closed to public entry. Follow the designated paths and information boards, and avoid climbing onto the mound or disturbing the archaeological remains. Respecting these restrictions helps protect the site for future research and for other visitors.
Rokudoyama Kofun Park: An area associated with the neighboring burial mound, with walking paths and viewpoints for exploring the surrounding landscape.
Museum of the Archaeological Institute of Kashihara: A useful destination for learning about artifacts and archaeological discoveries from the Kofun period.
Yata Nature Park: A natural area offering hiking routes and opportunities to enjoy the landscape around the Yata Hills.
Koizumi Otsuka Kofun and nearby Rokudoyama Kofun are important sites for understanding the development of elite burial traditions from the early to later Kofun periods. Their distinctive shapes, stone burial facilities, and archaeological discoveries reveal aspects of ancient engineering, social hierarchy, and cultural exchange.
Although much of the original landscape has changed, these burial mounds preserve valuable evidence of the communities that once flourished in the Nara Basin. By exploring the area respectfully and learning about the discoveries housed in local museums, visitors can gain a deeper appreciation of Japan’s ancient history and the people who shaped it.