Nyotaisan Kofun: A Scallop-Shaped Tomb on the Ota Plain
Most visitors who reach Nyotaisan Kofun arrive expecting something dramatic and instead find a low, wooded rise sitting quietly among houses and vegetable plots on the eastern edge of Ota. The mound stands only about seven metres tall. Walk its perimeter, though, and the scale registers. The burial mound runs 106 metres from end to end, and it was raised in the first half of the fifth century for someone of considerable rank.
The tomb belongs to a category Japanese archaeologists call a hotategai-gata kofun, or scallop-shaped tumulus: a large circular mound with a short rectangular platform attached to one side, so that the outline resembles a scallop shell. At Nyotaisan the round body measures 84 metres across, and the attached platform, known as a tsukuridashi, extends 18 metres wide and 16 metres long. Among tombs of this shape it ranks third in size in all of Japan, behind only Osahozuka in Miyazaki and Otomeyama in Nara. It is the largest example in the Kanto region.
A Tomb Built Beside Its Twin
The name "Nyotaisan" means roughly "female-body mountain." It is a counterpart to "Nantaisan," or "male-body mountain," the old popular name for the giant tomb a short distance to the southwest. That neighbour is Ota Tenjinyama Kofun, a keyhole-shaped tumulus 210 metres long and the largest ancient tomb in eastern Japan. Local people came to read the two mounds as a married pair, and the names stuck.
The pairing is more than folklore. The two tombs share the same orientation, their long axes pointing in the same direction across the plain. Archaeologists studying their proportions found that both were laid out using the same unit of measurement, a module of 25 Jin shaku (one shaku of roughly 24 centimetres, so about six metres). Design that precise and that consistent points to a single planning effort rather than two unrelated projects. Nyotaisan appears to have been completed slightly before Tenjinyama, around the middle of the fifth century.
Who lies inside is unknown, because the mound has never been excavated. The burial chamber is presumed to be a vertical pit cut into the top of the round mound, the usual method of the period. What the relationship was between the person buried here and the person buried in Tenjinyama also remains an open question. Comparable pairs exist elsewhere. At the Saitobaru cemetery in Miyazaki, a keyhole tomb and a scallop-shaped tomb stand together in much the same way, which suggests the arrangement carried meaning, perhaps marking rank or kinship.
Why the Tomb Is Protected
Nyotaisan Kofun was designated a National Historic Site on 8 April 1927, early in the Showa era and among the first waves of archaeological sites to receive national protection. The citation placed it in the category covering shell mounds, settlement remains, tombs and related ancient sites.
Two things make the tomb significant. The first is its size within its class. Scallop-shaped tombs were a deliberate alternative to the much larger keyhole tombs reserved for the highest elite, and a scallop tomb on this scale points to a person of real authority, if a step below the very top tier. The second is its state of preservation. The early survey that led to designation noted that the mound kept its moat and its distinctive form, and that it had survived unusually intact. More than a century later that still holds. The round body, the attached platform and the surrounding ditch are all legible on the ground.
The tomb belongs to a wider story as well. The Ota plain in the fifth century was the seat of a powerful local polity, and the rulers buried in tombs like Tenjinyama and Nyotaisan are thought to have held close ties to the Yamato court in the Nara basin. The line of monumental tombs across this part of Gunma marks one of the strongest regional powers in early eastern Japan.
What to Look For
The mound is easy to walk. The slopes are gentle, the height modest, and a path lets you climb to the top and circle the round body. Near the summit sits a small shrine, a reminder that long after its builders were forgotten the mound kept a place in local religious life.
Because the tomb has never been dug, there is no stone chamber to enter and no displayed artefacts on site. The interest lies in the shape itself. From the right angle the scallop outline becomes clear: the broad circular mound, then the lower rectangular platform pushing out from one side. That platform is only about a metre high, easy to miss unless you know to look for it.
Surface finds over the years have included haniwa, the unglazed clay cylinders and figures that ancient builders set around tombs. Plain cylindrical haniwa are thought to have ringed the mound, and at least one shield-shaped figurative haniwa has been recovered. The mound was once faced with fukiishi, a covering of fist-sized stones, though grass and trees now hide most of it. Standing on top, it helps to picture the plain below as it once was, dotted with mounds of this kind. Nyotaisan is the survivor that kept its full form.
Visiting the Area
Nyotaisan sits in the Uchigashima district on the eastern side of Ota, on a low plateau about 40 metres above sea level. It is roughly a 20-minute walk from Ota Station on the Tobu Isesaki Line, and a parking area is available for those arriving by car. A large shopping mall stands nearby, so the tomb is easy to fold into a wider day in the city.
The obvious companion visit is Ota Tenjinyama Kofun, a short walk to the southwest. Seeing the two together is the best way to grasp why they are described as a pair. Tenjinyama is the giant keyhole tomb; in the Edo period a large stone coffin of the "nagamochi" chest type was found there, and figurative haniwa shaped as houses, shields and birds have come from the site. To see excavated material up close, the Nitta-no-sho History Museum in the Serada district of Ota displays haniwa and other finds connected to these tombs.
One small reward for visitors: Ota distributes a collectable "kofun card" for the tomb, available at designated points around the city.
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Q&A
- What is a scallop-shaped kofun?
- It is a tomb built as a large round mound with a short rectangular platform attached to one side. The combined outline resembles a scallop shell, which gives the type its name. It is generally read as a form ranking just below the keyhole-shaped tombs of the highest elite.
- Can I go inside the tomb?
- No. The mound has never been excavated, so there is no open chamber. Visitors walk over and around the mound, but the burial itself remains sealed and unstudied.
- How is Nyotaisan related to Tenjinyama Kofun?
- The two tombs stand close together, share the same orientation, and were laid out with the same unit of measurement. This points to a single coordinated plan. Their popular names, "female-body" and "male-body" mountain, reflect a long tradition of treating them as a pair.
- How long does a visit take?
- Walking the mound and circling its base takes around 20 to 30 minutes. Combined with nearby Tenjinyama Kofun, allow about half a day for both.
- When was the tomb built?
- Around the first half to middle of the fifth century, slightly earlier than Tenjinyama. It was designated a National Historic Site in 1927.
Basic Information
| Name | Nyotaisan Kofun |
|---|---|
| Location | Uchigashima-cho 1506-1 and others, Ota City, Gunma Prefecture |
| Designation | National Historic Site |
| Designation date | 8 April 1927 (Showa 2) |
| Type | Scallop-shaped tumulus (round mound with projection) |
| Period | First half to middle of the 5th century |
| Scale | Mound length 106 m; round mound diameter 84 m; height 7 m; projection 18 m wide and 16 m long |
| Access to site | Open grounds; no enclosed structures; parking available |
| Getting there | About 20 minutes on foot from Ota Station (Tobu Isesaki Line) |
References
- Database of National Cultural Properties, Agency for Cultural Affairs: Nyotaisan Kofun
- https://kunishitei.bunka.go.jp/bsys/maindetails/401/526
- Ota City official website, Cultural Properties Division: Nyotaisan Kofun
- https://www.city.ota.gunma.jp/page/4524.html
- Ota City Tourism and Products Association: Tenjinyama Kofun cultural-property route
- https://www.ota-kanko.jp/course/tenjinyama/
- Wikipedia (Japanese): Nyotaisan Kofun
- https://ja.wikipedia.org/wiki/女体山古墳
Last verified: 2026.05.22