Its Place within the Soja Kofun Group
The Soja Kofun Group is one of eastern Japan’s important clusters of ancient burial mounds. Six major tombs are distributed across an area approximately 4 kilometers from north to south.
One of the group’s most important features is that it preserves the transition from the keyhole-shaped burial mounds of the late fifth and sixth centuries to the square burial mounds that became characteristic of the seventh century.
Jaketsuyama Kofun is believed to have been constructed in the late seventh century, after Hōtōyama Kofun. It is considered the final large square tomb constructed within the Soja Kofun Group.
Even after the traditional keyhole-shaped tombs had disappeared, a monument of considerable size with an exceptionally sophisticated stone burial chamber was still constructed here. This suggests that the Soja area remained an important political center in the ancient Kamikōzuke region.
A Majestic Three-Tiered Square Mound
Square Mound with Carefully Constructed Stone Covering
Jaketsuyama Kofun is square in plan, measuring approximately 44 meters on each side and about 5 meters high. Its central axis is rotated approximately 13 degrees east from north. This orientation has been associated by researchers with the alignment of the nearby ancient provincial government offices of Ueno Province and Sanno Haiji Temple.
The mound is believed to have been constructed in three tiers and was covered with fukiishi, stones traditionally placed over ancient burial mounds.
On the middle embankment and outer surfaces, relatively flat stones were carefully laid horizontally in an alternating pattern. These remains reveal the considerable attention given to the construction and appearance of the mound.
Double Moats and an Intermediate Embankment
Another distinctive feature is the presence of two surrounding moats: an inner moat immediately around the mound and an outer ditch.
Including the moats, the entire monument extends approximately 82 meters on each side. The inner moat is about 11 meters wide on average and between 1.1 and 1.9 meters deep. The intermediate embankment appears to have been formed by cutting and shaping the natural ground.
A medieval text known as the Shintōshū, compiled in the latter half of the 14th century, describes a “Rock Chamber of the Snake Mound” on an island in a pond called Jaketsu Pond. This has led researchers to suggest that the surrounding moat may once have been used as a pond.
The changing role of the site over the centuries adds another fascinating layer to the history of Jaketsuyama Kofun.
An Exceptionally Refined Stone Burial Chamber
A Distinctive Layout without a Conventional Passageway
The burial facility is a side-entry stone chamber with side extensions, opening toward the south-southwest.
Unlike many other ancient Japanese stone burial chambers, it does not have a clearly defined long entrance passage. Instead, a small intermediate area and an open forecourt connect directly to the main chamber.
The main chamber measures approximately 3 meters long, 2.6 meters wide, and 1.8 meters high. Although relatively compact, its construction demonstrates an exceptionally high level of stone-working technology.
A Carefully Engineered Chamber Built from Seven Cut Stones
The chamber was constructed from carefully cut pyroxene andesite. Remarkably, the two side walls, rear wall, ceiling stone, two entrance stones, and capstone are each made from a single stone, creating a structure of only seven principal cut stones.
The entrance stones were carefully shaped to resemble architectural pillars and a lintel, while evidence suggests that stone door slabs were once used to seal the chamber.
The surfaces of the stones show traces of light polishing and plaster. Inside the chamber, a cut stone made from Ushibuse sandstone is believed to have served as a platform for the coffin.
The sophisticated workmanship has been compared with the stone foundation of the pagoda at the nearby Sanno Haiji Temple ruins. This suggests a continuity of advanced stone-working techniques from the Kofun period into the subsequent Asuka period.
A Close Relationship with Hōtōyama Kofun
Hōtōyama Kofun lies approximately 120 meters west of Jaketsuyama Kofun. The protected areas of the two tombs are separated by only about 20 meters, suggesting a close historical relationship.
There are, however, notable differences between the two monuments. Hōtōyama Kofun has a single surrounding moat, whereas Jaketsuyama Kofun has an intermediate embankment and an additional outer ditch.
The orientation of Jaketsuyama Kofun is also closer to true north. This change in alignment has attracted attention because it may reflect changing concepts of spatial orientation that became evident from around the middle of the seventh century onward.
Historical Discoveries and Preservation
Jaketsuyama Kofun appears in medieval written sources, and during the Kanbun era of the 17th century, an image of a snake was carved into the rear wall of its stone chamber.
An archaeological investigation carried out in 1975 as part of a site preservation project confirmed the square shape of the mound.
Since the 2000s, further surveys have been conducted to determine the extent of the historic site, along with detailed three-dimensional measurements of the mound and stone chamber. Additional land was included in the designated historic site in 2021, followed by the reorganization and expansion of the designation as the Soja Kofun Group in 2024.
The Soja Kofun Group and Ancient Ueno Province
The ruins of Sanno Haiji Temple, believed to have been founded in the latter half of the seventh century, lie approximately 1 kilometer southwest of the Soja Kofun Group.
The former Ueno Provincial Government Office and the ruins of the Ueno Provincial Buddhist Temples are located approximately 2 kilometers to the south. Together, these archaeological sites demonstrate that the Soja area was an important political and cultural center of ancient Ueno Province.
Three large square tombs in the group—Atagoyama, Hōtōyama, and Jaketsuyama Kofun—were constructed after the end of the keyhole-tomb tradition. They provide important evidence of changes in burial customs among regional elites and are also considered valuable evidence of the strong relationship between local rulers and the Yamato political authority of ancient Japan.
Visiting Jaketsuyama Kofun
Today, Jaketsuyama Kofun has been preserved and is open to visitors. The site can be explored freely, allowing visitors to walk around the square mound and appreciate the remains of its ancient construction.
The tomb is approximately a 20-minute walk from Gunma-Sōja Station on the JR Jōetsu Line. Nearby Hōtōyama Kofun and Soja Futagoyama Kofun can also be visited, making the area suitable for a walking tour through the ancient burial sites of the Soja Kofun Group.
Standing on the quiet plateau, visitors can look across the surrounding landscape and imagine the scenery once seen by the powerful leaders buried here.
Seeing the traces of the stone-covered mound and the remarkably precise construction of the burial chamber up close offers a vivid introduction to the history of the late seventh century, a crucial period in the formation of the ancient Japanese state.
The Final Monument of the Soja Kofun Group
Jaketsuyama Kofun is a valuable archaeological site that demonstrates both the advanced stone-working techniques and political circumstances of the final stage of the Kofun period.
Its square shape represents an important change from the earlier keyhole-shaped tombs, while its sophisticated stone chamber reflects the high level of technology available to the elites of the period.
As the final large burial mound in the Soja Kofun Group, Jaketsuyama Kofun brings together the story of this remarkable ancient landscape. A visit provides an opportunity to encounter more than 1,300 years of history and to consider the people, beliefs, technology, and political structures that shaped ancient Gunma.