The Location and Origin of the Name
A Giant Ancient Monument Close to the City Center
Ota Tenjinyama Kofun was built near the eastern edge of a plateau extending across eastern Gunma Prefecture, at an elevation of approximately 40 meters.
It is located about 1 kilometer east of Ota Station on the Tobu Isesaki Line, making it unusual in that such a massive ancient burial mound survives so close to a modern urban area.
The Tenmangu Shrine and the Name “Tenjinyama”
A small Tenmangu shrine stands near the narrow central section connecting the two parts of the mound. The presence of this shrine gave rise to the name Tenjinyama Kofun, meaning roughly “Tenjin Mountain Burial Mound.”
Although modern roads and railway lines now pass around the site, much of the original mound and its inner moat remain comparatively well preserved.
A prefectural road runs along the base of the rear circular section, while the Tobu Koizumi Line crosses the area near the outer moat of the front section. Despite these modern intrusions, the surviving earthworks provide a clear impression of the enormous scale of the ancient monument.
The Largest Keyhole-Shaped Burial Mound in Eastern Japan
An Extraordinary Scale
The most striking feature of Ota Tenjinyama Kofun is its sheer size. The mound measures approximately 210 meters from end to end, making it the largest kofun of its type in eastern Japan.
It is also the only keyhole-shaped burial mound in the Kanto region exceeding 200 meters in length. Its dimensions suggest that the person buried here belonged to a powerful ruling elite with considerable political and economic resources.
The Three-Tiered Burial Mound
The mound was constructed in three levels, with both the front and rear sections carefully shaped into stepped terraces.
The rear circular section has a diameter of approximately 120 meters and reaches about 16.5 meters in height. At the top is a relatively flat area approximately 24 meters in diameter.
The front section is approximately 126 meters wide and about 12 meters high. Together, these enormous earthworks create a monument on a scale that remains impressive even after approximately 1,500 years.
Stone Covering Known as Fuki-ishi
The outer surface of the mound was covered with fuki-ishi, or stones laid over the earthwork. Many of these stones came from riverbeds within the Watarase River watershed.
When the kofun was newly constructed, the stones would have reflected sunlight across the surface of the mound. The monument may therefore have presented a striking and highly visible appearance, emphasizing the status of the person buried within it.
Double Moats Surrounding the Kofun
A Monumental Sacred Landscape
The enormous burial mound is surrounded by an equally impressive system of double moats. An embankment known as a nakatei separates the inner and outer moats.
Including the moats, the entire monument measures approximately 364 meters in length and 288 meters in width. The site therefore extends far beyond the visible mound itself.
Distinctive Shapes of the Inner and Outer Moats
The inner moat has a distinctive shield-like shape, while the outer moat has a horseshoe-like shape. These unusual forms provide clues about the sophisticated planning involved in constructing the monument.
The moats are not especially deep by modern standards. However, they are believed to have held water when the kofun was first constructed, emphasizing the sacred character of the burial complex and helping to distinguish it from the surrounding landscape.
Discoveries That Reveal Ancient Japanese Culture
A Wealth of Haniwa
Archaeological investigations have uncovered a variety of artifacts from Ota Tenjinyama Kofun. Among the most important are numerous examples of haniwa, clay figures and objects placed around kofun during ancient burial ceremonies.
Excavated examples include cylindrical haniwa, as well as haniwa representing houses, shields, and other objects. These artifacts provide valuable clues about the rituals, beliefs, and material culture of the people who constructed the kofun.
Animal-Shaped Haniwa
Particularly interesting are animal-shaped haniwa depicting creatures such as water birds and chickens.
The discovery of the head of a water-bird haniwa is especially noteworthy. Such representations may have been connected with rituals or beliefs surrounding the burial ceremony, although the precise meaning remains a subject of archaeological study.
The Stone Coffin Discovered During the Edo Period
Historical records indicate that a house-shaped stone coffin, known as a nagamochi-gata sekkan, was discovered at the kofun during the Edo period.
This type of stone coffin is particularly associated with elite tombs in the Kinai region, the area around present-day Nara and Osaka that became the political center of the ancient Yamato state.
Its presence has therefore led researchers to consider the possibility that the person buried at Tenjinyama had strong connections with the Yamato royal authority.
When Was the Kofun Built?
A Powerful Ruler of Ancient Keno
Based on the characteristics of the excavated haniwa and pottery, Ota Tenjinyama Kofun is generally estimated to have been constructed during the first half of the 5th century, in the middle of Japan’s Kofun period.
At that time, the region now known as Gunma was part of an area called Keno, which was home to powerful regional political groups.
Given the enormous scale of the kofun and the type of stone coffin associated with it, the buried individual is thought to have been a major leader of the Keno region. The identity of the individual, however, has not been definitively established.
Relationship with Nyotaiyama Kofun
Approximately 300 meters northeast of Tenjinyama Kofun is Nyotaiyama Kofun, a distinctive scallop-shell-shaped burial mound.
The two kofun share similarities in their orientation and design, leading researchers to suggest that they may have been constructed according to a common plan.
The relationship between the two monuments suggests that this area may once have formed a large burial landscape associated with the region’s ancient ruling elite.
The History of Research and Preservation
Known Since the Edo Period
Tenjinyama Kofun has been recognized as an important monument for centuries. Records from the Edo period already mention the discovery of the stone coffin.
Modern academic research progressed during the 20th century, and the kofun was included in the archaeological survey “Comprehensive Survey of Kofun in Jomo” in 1938.
Designation as a National Historic Site
In 1941, the historical importance of the kofun was formally recognized when it was designated a National Historic Site.
Subsequent surveying and archaeological investigations have revealed more about the arrangement of haniwa, the outer moat, and other features of the burial complex. Research continues today, and the site remains an important source of information about the politics and culture of Kofun-period Japan.
Enjoying Tenjinyama Kofun as a Tourist Attraction
Ota Tenjinyama Kofun is valuable not only for its archaeological importance but also as a place for a quiet walk through an ancient landscape.
The relatively open surroundings allow visitors to appreciate the enormous dimensions of the burial mound and imagine how dramatically it would have dominated the landscape when it was first built.
Seasonal Flowers
In summer, the bright orange flowers of Lycoris sanguinea, commonly known as the red spider lily relative or fox’s candle, bloom around the kofun. The vivid flowers create a striking contrast with the green vegetation surrounding the ancient mound.
The seasonal scenery is particularly attractive to photographers and visitors who enjoy combining historical sightseeing with nature.
Nearby Historical Attractions
Ota City has many other historic destinations, including the Nitta Kanayama Castle Ruins and Daikoin Temple. Combining these sites with Tenjinyama Kofun makes it possible to explore several different periods of Gunma’s history in a single itinerary.
Related Museum
The Ota Municipal Nitta Manor History Museum in Serada-cho displays artifacts excavated from Tenjinyama Kofun and provides additional information about the ancient and medieval history of the surrounding area.
Access
Ota Tenjinyama Kofun is approximately a 15-minute walk east from Ota Station, served by the Tobu Isesaki, Kiryu, and Koizumi Lines. The route follows Prefectural Route 2, also known as the Togoku Cultural and Historical Road.
A Monument to the Power of Ancient Japan
Ota Tenjinyama Kofun is an extraordinary ancient monument and the largest keyhole-shaped burial mound in eastern Japan. Its enormous mound, double moats, extensive haniwa collection, and distinctive stone coffin provide valuable evidence of the political power and cultural traditions of Kofun-period Japan.
Its location close to central Ota makes it especially accessible for visitors interested in ancient Japanese history. At the same time, the surrounding greenery and open landscape make it an enjoyable destination for a leisurely walk.
For travelers exploring Gunma, a visit to this remarkable kofun offers a chance to step away from the modern city and contemplate a world that existed approximately 1,500 years ago, when enormous burial monuments were built to commemorate the region’s most powerful leaders.