The Largest Tomb in the Soja Kofun Group
The Soja Kofun Group consists of major burial mounds distributed across an area approximately 4 kilometers from north to south around Soja-machi in Maebashi. Six major tombs are currently recognized.
Among them, Soja Futagoyama Kofun is one of the largest, with a total mound length of approximately 89.9 meters.
Structure and Scale of the Burial Mound
The tomb has a keyhole-shaped design, consisting of a rounded rear section and a rectangular front section. The front section faces west. The mound was constructed in two tiers, with stone coverings known as fukiishi and cylindrical haniwa figures believed to have once stood around the mound.
A moat also surrounded the mound, emphasizing the status and grandeur of the person buried here.
The rear circular section measures approximately 44 meters in diameter and 7 meters high, while the front section is approximately 61 meters wide and 8 meters high.
Viewed from the front toward the rear mound, the monument has the appearance of a gentle artificial hill. Its enormous scale provides a vivid impression of the sophisticated construction techniques available to ancient communities in the region.
Two Stone Burial Chambers
One of the most distinctive features of Soja Futagoyama Kofun is that it contains two side-entry stone burial chambers, one in the rear circular section and another in the front section. Both chambers open toward the south.
The Rear Burial Chamber: One of the Largest in Gunma
The burial chamber in the rear section is approximately 9.4 meters long, with an inner chamber measuring about 6.88 meters in length and a rear wall approximately 3.40 meters wide.
It is one of the largest stone burial chambers discovered in Gunma. The stones are hornblende andesite produced by volcanic activity at Mount Futatsudake on Mount Haruna. Carefully cut stones with five finished faces were laid in an alternating pattern, demonstrating a high level of masonry skill.
Although the original ceiling stones have collapsed, the size and sophisticated construction of the chamber provide important evidence of the power held by the regional ruling elite of ancient Kamikōzuke.
The Front Burial Chamber and an Elegant Kabutsuchi Sword
The burial chamber in the front section is approximately 8.76 meters long, with an inner chamber measuring about 4.27 meters. Unlike the rear chamber, it was constructed using irregularly arranged natural stones.
An excavation carried out in 1819 uncovered human remains and numerous burial goods.
Among the most remarkable objects was a beautifully decorated Kabutsuchi-no-Tachi, an ancient Japanese sword distinguished by its elaborate pommel decoration. The sword itself has not survived or its present location is unknown, but detailed illustrations made at the time reveal its sophisticated design.
Other discoveries included iron swords, small knives, magatama curved beads, ornaments, and Sue ware, including a long-necked jar with a pedestal. Some of these archaeological materials are now held by the Tokyo National Museum.
Construction Date and Historical Significance
Soja Futagoyama Kofun is believed to have been constructed during the late sixth century. Within the Soja Kofun Group, it is considered to have been built after Oyam Kofun and before Atagoyama Kofun.
It belongs to an important period when the traditional keyhole-shaped tomb was gradually giving way to square burial mounds. The sequence of tombs in the surrounding area provides valuable evidence of this major transition in ancient Japanese burial culture.
The area around the Soja Kofun Group was also an important political and cultural center of ancient Kozuke. The remains of Sanno Haiji Temple, founded in the latter half of the seventh century, as well as the former provincial government offices and provincial Buddhist temples of the Nara period, are located nearby.
The fact that powerful leaders continued to be buried in this area over several centuries reflects the importance of the region’s political structure and its connections with the Yamato court, the central political power of ancient Japan.
The Tomb’s Historical Connection with Imperial Mausoleums
During the early Meiji period, when tombs throughout Japan were being investigated and classified, Soja Futagoyama Kofun was submitted as the possible burial place of Prince Toyoki-irihiko, traditionally described as the eldest son of Emperor Sujin.
It was temporarily designated as an imperial mausoleum, but the designation was naturally discontinued the following year due to circumstances within the local community.
The identity of the person buried in the tomb remains uncertain. Archaeological evidence, however, suggests that it was the burial place of a powerful leader or elite clan from ancient Kamikōzuke.
The Development of the Soja Kofun Group
On February 21, 2024, the former National Historic Sites of Futagoyama Kofun, Hōtōyama Kofun, and Jaketsuyama Kofun were combined with Tomiyama Kofun and Atagoyama Kofun. The designation was renamed the “Soja Kofun Group.” The protected area was further expanded in 2025.
Tomiyama Kofun — Late 5th Century
A keyhole-shaped burial mound approximately 88 meters long.
Oyam Kofun — Early 6th Century
A keyhole-shaped burial mound approximately 76 meters long.
Soja Futagoyama Kofun — Late 6th Century
A keyhole-shaped burial mound approximately 90 meters long and a designated National Historic Site.
Atagoyama Kofun — Early 7th Century
A square burial mound approximately 56 meters on each side and a designated National Historic Site.
Hōtōyama Kofun — Mid-7th Century
A square burial mound approximately 60 meters on each side and a designated National Historic Site.
Jaketsuyama Kofun — Late 7th Century
A square burial mound approximately 40 meters on each side and a designated National Historic Site.
The sequence of these tombs is particularly valuable because it allows visitors and researchers to see the gradual transition from the keyhole-shaped tombs of earlier periods to the square tombs that became more common later. The group is therefore an important site for understanding the development of Kofun-period culture in eastern Japan.
Visiting Soja Futagoyama Kofun
Today, Soja Futagoyama Kofun has been preserved as a historic site where visitors can walk around the ancient mound and surrounding grounds.
Standing on the mound provides views across the Maebashi Plateau and toward the Mount Haruna area. The landscape offers an opportunity to imagine the scenery that ancient leaders may once have looked across more than 1,400 years ago.
Explore the Ancient History of Soja
The nearby Maebashi City Soja History Museum introduces the history of the local kofun and displays archaeological discoveries from the surrounding area.
Visitors can also combine their visit with the nearby Sanno Haiji Temple ruins and Ueno Kokubunji Temple ruins. Exploring these sites together provides a broader understanding of the political, religious, and cultural landscape of ancient Kozuke.
A Place to Experience the Ancient History of Eastern Japan
Soja Futagoyama Kofun is far more than a massive mound of earth. It is a tangible reminder of the political development of eastern Japan during the period when the foundations of the ancient Japanese state were taking shape.
Built in the late sixth century, the tomb reflects the authority, beliefs, and social organization of the powerful leaders who governed this region, as well as their connections with the Yamato political center.
When visiting Maebashi, consider exploring the Soja Kofun Group and its surrounding historical sites. Walking across the quiet ancient mound offers a rare chance to encounter the history of more than a thousand years ago while enjoying the landscape of present-day Gunma.