Ryukakuji Burial Mounds and Iwaya Burial Mounds

Ryūkakuji Kofun Group and Iwaya Kofun: Japan's Largest Late-Period Square Tumulus

On the wooded plateau north of Inba-numa lake, a flat-topped earthen pyramid rises in three terraced steps. Each side measures roughly 78 meters, and the mound climbs 13.2 meters from base to summit. This is Iwaya Kofun—the largest square tumulus ever built in the Late Kofun period anywhere in Japan, and, by volume, the second-largest square tumulus in the country across all eras.

Iwaya is the most prominent member of the Ryūkakuji Kofun Group, a cluster of 115 burial mounds spread across the border of Sakae Town in Inba District and the city of Narita in Chiba Prefecture. The group was raised between the sixth and seventh centuries, in the closing chapter of Japan's tumulus age, just before the country reorganized itself around Buddhist temple-building and the codified law of the Ritsuryō state.

Two stone chambers open on the south face of Iwaya Kofun, set side by side and built from soft sandstone studded with fossil shells. Old records show that those chambers were already standing open by 1591, which is one reason that, despite the tomb's size, not a single grave good has been recovered from inside.

A Burial Ground at the Edge of the Kofun Age

The Ryūkakuji Kofun Group sits on a narrow plateau about thirty meters above sea level, stretching from the hamlet of Sakanao and the village of Ryūkakuji in Sakae Town to the Ōtake district of Narita. The terrain pushes against the northeastern shore of Inba-numa, in a landscape once defined by waterways feeding into the Tone River system.

Of the 115 mounds identified to date, archaeologists count 37 keyhole-shaped tumuli (zenpōkōen-fun), 71 round mounds (enpun), and 6 square mounds (hōfun). The ratio of keyhole tombs is unusually high compared with other clusters from the same era—a sign that the local lineage retained considerable independence and prestige well into the seventh century.

The mounds were not built in a single push. The earlier ones, lining the rim of the plateau where the land drops toward the lake, are sixth-century round and keyhole tombs of modest size. Larger keyhole tombs and round mounds appear in the central zone, around what is now the Fudoki-no-Oka Museum of Chiba Prefectural Boso-no-Mura. The youngest mounds—the seventh-century square tumuli, including Iwaya—cluster at the eastern end of the plateau, at the head of a small valley draining into the Negina River.

That shift, from keyhole to square, mirrors the political transformation underway across Japan. As the Yamato court extended its grip eastward, regional chieftains adopted the architectural vocabulary of the imperial center. Iwaya Kofun, built after the great keyhole tomb of Sengen-yama, marks the end of that lineage in Chiba.

Why a Single Hilltop Became a Nationally Protected Site

Iwaya Kofun alone was designated a National Historic Site on January 27, 1941, recognized at the time for the unusual scale of its earthworks and the rare construction of its twin chambers. For nearly seven decades that single tomb carried the designation by itself.

That changed on February 12, 2009, when the Agency for Cultural Affairs expanded the protection to encompass 93 of the 115 mounds along with the surrounding terrain, formally renaming the property "Ryūkakuji Kofun Group and Iwaya Kofun." The protected area now covers roughly 445,000 square meters, making it the largest National Historic Site by area anywhere in Chiba Prefecture.

The reasoning behind the expansion turns on context. Excavated mokkan wooden tablets at the Heijō Palace site in Nara have identified the eighth-century district chief of Hanyū as a member of the Ōufube clan. Many archaeologists now think the same family—or its direct ancestors, the Inba-no-kuni-no-miyatsuko—commissioned both the kofun group and the nearby Ryūkakuji temple, founded in the late seventh century. Read together, the burial mounds, the ancient road network, the Hanyū county office ruins, and the Hakuhō-era temple trace a single arc: a regional power transitioning from tumulus-builders to administrators of the Ritsuryō state. Few places in eastern Japan preserve that transition so completely.

Walking the Mounds: What to See on the Ground

Iwaya Kofun (Mound 105)

From the south, the tomb reads as a stepped wall of earth and forest, twin chamber mouths set into the lower terrace about nine meters apart. Visitors can circle the mound on a level path that traces the outer moat. The eastern chamber is the larger of the two, about seven meters deep, two meters wide, and three and a half meters high; the western chamber is shorter and narrower. Only the eastern chamber is currently accessible to look into.

The masonry repays close attention. Most of the blocks come from the Kioroshi shell layer, a band of soft fossiliferous sandstone that crops out near Inba-numa. The stones were sawn to roughly the size of large bricks and laid in courses, alternating like a wall. A few of the ceiling stones, however, are mica schist quarried at the Tsukuba range across the Kantō Plain, the same material used a generation earlier in the chamber of Sengen-yama Kofun. After Iwaya, that imported schist disappears from the local building tradition—a small detail that lets archaeologists order the tombs in time.

Mound 101: The Reconstructed Round Tomb

A short walk south of Iwaya, in the Narita-side portion of the protected area, sits Mound 101. It is a circular tomb roughly 25 meters across and 3.6 meters high, ringed by a double moat. Excavation recovered an extraordinary array of figural haniwa—warriors, horses, and human figures—and the mound has been restored to something like its original appearance, with cylindrical haniwa standing in rows around the rim and figures arranged along the access platform. It is the easiest place in the group to picture what these tombs looked like when freshly built.

Sengen-yama Kofun (Mound 111)

Tucked deeper into the woods, Sengen-yama is the largest keyhole tomb in the group at 78 meters long and the last keyhole tomb ever built in Chiba. Excavations have yielded gilt-bronze and silver crown ornaments and gilt-bronze horse fittings, finds rare enough that they carry their own prefectural cultural property designation. The mound sits outside the most frequented part of Boso-no-Mura, and the path takes some looking for, but the scale of the chamber walls makes the detour worthwhile.

Around the Site

Most of the protected area falls inside Chiba Prefectural Boso-no-Mura, an open-air living-history museum that occupies the same plateau. The museum's Fudoki-no-Oka district reconstructs prehistoric pit dwellings and displays excavated material from across Chiba; the adjoining Furusato-no-Waza zone recreates an Edo-period merchant street, a samurai house, and three regional farmhouses, with hands-on workshops in crafts ranging from soba-making to indigo dyeing.

The old auditorium of the Gakushūin Primary School, an Important Cultural Property in its own right, stands a short walk from Mound 101. To the north lies Ryūkakuji temple, founded in the late seventh century and home to a seated Yakushi Nyorai bronze that is among the oldest surviving Buddhist sculptures in eastern Japan. A combined visit to temple, kofun, and museum makes a full day, easily reached from Narita Airport by bus and train.