The Gion-Nagasuka Kofun Tumulus Group is a collection of ancient burial mounds located mainly on the alluvial plain around the lower reaches of the Obitsu River in Kisarazu City, Chiba Prefecture. The tumuli were constructed from around the mid-5th century through the 7th century.
Because of its historical background and geographical setting, the group is regarded as an important archaeological site representing the ancient history of the Kisarazu area and the Boso Peninsula.
The tumuli were built mainly on a series of ancient sand dunes in the lower Obitsu River basin. Four lines of former sand dunes running roughly north to south have been identified in the area.
Many of the burial mounds were constructed on the second, third, and fourth dune lines from the coast, while some were also built in nearby hilly areas. Their construction dates range from the mid-5th century to the 7th century.
The tombs are believed to have been associated primarily with the Makuta no Kuni no Miyatsuko, a regional leader who governed the area around the Obitsu River under the ancient Japanese political system.
The Gion-Nagasuka Kofun Tumulus Group was heavily affected by urban development, and many of the original burial mounds have disappeared.
Even some of the surviving tumuli have been substantially altered and no longer retain their original forms. As a result, it is difficult to reconstruct the complete appearance of the ancient cemetery.
Nevertheless, surviving sites such as Kinreizuka Kofun, together with the valuable burial goods discovered there and at other sites, provide important evidence of the power and status of the regional leaders who were buried in these tombs.
The earliest known burial mound in the Gion-Nagasuka group is believed to be Takayanagi Choshizuka Kofun, which was probably constructed during the second quarter of the 5th century.
This keyhole-shaped tomb, known in Japanese as a zenpō-kōen-fun, is estimated to have had a total mound length of approximately 130 to 150 meters. It was one of the largest burial mounds on the Boso Peninsula at the time.
Other large tumuli, including Gion Otsukayama Kofun, were subsequently built in the surrounding hilly areas. Their impressive scale indicates the presence of powerful local leaders.
Tumulus construction in the Gion-Nagasuka area declined between the end of the 5th century and the middle of the 6th century.
A similar interruption can be observed in other parts of the Boso Peninsula. One possible explanation is that political instability within the Yamato court may have affected the activities and status of regional rulers, although the exact reasons remain uncertain.
Construction resumed around the middle of the 6th century. Sakamorizuka Kofun and Inarimori Kofun are among the representative examples from this period.
The period from the late 6th century to the beginning of the 7th century can be regarded as the peak of construction at the Gion-Nagasuka Kofun Group.
During this period, large keyhole-shaped tombs, as well as medium- and small-sized burial mounds, were constructed one after another. One of the most important was Kinreizuka Kofun, which had a mound approximately 100 meters long.
Its burial chamber had not been looted when it was excavated, allowing archaeologists to recover an exceptionally rich collection of burial goods. These artifacts provide valuable evidence of the authority and wealth of the regional ruler buried there.
From the beginning of the 7th century, the construction of keyhole-shaped tombs came to an end, and square burial mounds became more common.
One notable example is Matsumo Kofun, a square tumulus measuring approximately 44 meters on each side. Its overall scale was even larger when the surrounding moat is taken into account, and it is believed to have been constructed during the first half of the 7th century.
By the latter half of the 7th century, Kazusa Daiji Temple was established near the Gion-Nagasuka Kofun Group. This change illustrates a broader transformation in the way political and religious authority was expressed: monumental temples gradually replaced burial mounds as important symbols of power.
With an estimated mound length of 130 to 150 meters, Takayanagi Choshizuka Kofun was one of the largest burial mounds on the Boso Peninsula. It is believed to have been constructed during the second quarter of the 5th century.
Gion Otsukayama Kofun is estimated to have had a mound approximately 100 meters long. Excavations conducted in 1891 uncovered valuable artifacts, including gilt-bronze armor and a decorated bronze mirror known as a four-Buddha and four-beast mirror.
Kinreizuka Kofun had a mound approximately 100 meters long. Because its burial chamber had remained unlooted, archaeologists discovered a remarkable collection of burial goods, including golden bells and elaborately decorated swords.
These artifacts offer important insights into the wealth, social status, craftsmanship, and connections of the powerful leader buried within the tomb.
Matsumo Kofun is a square burial mound measuring approximately 44 meters on each side. Including its surrounding moat, the overall structure was even larger. It is believed to have been constructed during the first half of the 7th century.
The Gion-Nagasuka Kofun Tumulus Group is an important archaeological site for understanding the history and culture of the Kofun period on the Boso Peninsula.
Although urban development has resulted in the loss or alteration of many of the original burial mounds, surviving remains and excavated artifacts—particularly those associated with Kinreizuka Kofun—provide valuable clues about the authority of ancient local rulers, their relationship with the Yamato court, and connections between different regions of ancient Japan.
The tumulus group remains an important part of Chiba Prefecture’s historical heritage and provides a fascinating glimpse into the political and cultural landscape of Japan more than a thousand years ago.