Higashinomiya Kofun

Higashinomiya Kofun: A Fourth-Century Ruler's Tomb Above the Kiso River

In March 1973, grave robbers broke into a wooded hilltop overlooking the Kiso River in what is now Inuyama City, Aichi Prefecture. The damage they inflicted triggered an emergency excavation that summer, and when archaeologists opened the chamber beneath, they encountered something extraordinary: eleven bronze mirrors, jasper bracelets, jade beads, and dozens of iron weapons, all undisturbed since the early Kofun period. The site is now known as Higashinomiya Kofun (東之宮古墳), one of the oldest large burial mounds in Aichi and a rare survivor that has endured more than seventeen centuries with its shape largely intact.

The tomb sits on the western summit of Hakusanbira-yama, a chert ridge rising to 143 meters above sea level. Below, the Kiso River bends past the town of Inuyama, and on a clear day the view extends across the Nobi Plain toward the volcanic peaks of the Hida range. The mound was built here deliberately, on a position commanding water, valley, and sky.

A "Square-Front, Square-Back" Tomb of the Early Kofun Period

Higashinomiya Kofun belongs to a less common variety of keyhole-shaped tomb called zenpō-kōhōfun (前方後方墳), literally "front-square, back-square." Unlike the more familiar round-rear keyhole tombs that became the standard form for Yamato rulers, this design pairs two rectangular platforms: a larger one for burial and a smaller one in front, possibly for ritual. The form was favored in certain regions of central and eastern Japan during the early Kofun period, and Higashinomiya is among its finest examples.

The mound runs about 72 meters from end to end. The rear square measures roughly 48 by 49 meters and rises to a height of 9 meters; the front section is 24 meters long, 43 meters wide, and 7 meters tall. The builders did not simply pile up a hill. They first cut the chert bedrock flat to create a level platform, then hauled in earth from the base of the mountain and shaped the mound on top, finishing the surface with a layer of cladding stones — mainly local red chert, supplemented by rounded river cobbles likely carried up from the Kiso River. The rear section was built in two stepped tiers, a detail confirmed only in surveys conducted from 2005 onward.

Inside the rear summit lay three burial pits, two of them set parallel along the mound's central axis with a third on the front platform. Only one has been excavated. That chamber, of the type called a vertical pit stone chamber (竪穴式石槨), measures 4.8 meters long and roughly 96 centimeters wide. Its walls were stacked from alternating slabs of tuff and granite laid on edge; small pebbles filled the inner face, painted over with red iron-oxide pigment called bengara. Seven flat tuff stones formed the ceiling, sealed above with a careful layer of clay. The wooden coffin that rested on the chamber's clay floor has long since rotted away, but its outline survives.

Why It Was Designated a National Historic Site

The Agency for Cultural Affairs designated Higashinomiya Kofun a National Historic Site on July 19, 1975, less than two years after the emergency excavation. The mound's surroundings were added to the designation on February 22, 2010. The grave goods themselves were designated Important Cultural Property on June 15, 1978, and are held in the collection of the Kyoto National Museum.

Three factors place the site in the top rank of early Kofun period burials. First is the sheer quantity and quality of the offerings: eleven bronze mirrors recovered from a single chamber rivals what archaeologists find in the much larger royal tombs of the Kinai region. Second is the survival of the mound itself, which has resisted earthquakes and heavy rainfall for some seventeen hundred years with no major collapse. Third is the site's position on the cultural map. The presence of triangular-rimmed deity-and-beast mirrors (三角縁神獣鏡) connects whoever was buried here to the network of regional leaders aligned with the emerging Yamato polity in the Kinai region, while four mirrors of a type called jinbutsu-kinjū-mon-kyō (人物禽獣文鏡), made domestically and found only in the Nobi Plain, suggest that the same person also commanded local craft traditions of considerable sophistication.

Most authorities now date the construction to the late 3rd or early 4th century, making Higashinomiya one of the earliest large kofun in Aichi Prefecture and a key reference point for understanding the transition into the Kofun period in the Nobi Plain.

Highlights for Visitors

The most photographed view is the mound itself from the north side, where the original square-back profile is best preserved. Walk slowly around the perimeter and the two-tiered terrace of the rear platform becomes apparent. An interpretive panel on the north side shows the eleven mirrors at actual size — useful because the originals are kept in Kyoto, not at the site.

The path up the mountain begins at the Naritasan entrance and climbs gently through cypress and broadleaf forest. The trail itself doubles as a geology lesson: outcrops of folded red chert appear along the way, the same stone that builds the underlying ridge and the mound's cladding. The summit clearing where the kofun sits offers a wide outlook over the Kiso River, with Inuyama Castle visible on the next hilltop to the west.

One observation worth timing a visit for: on the day of the winter solstice, the rising sun aligns precisely with the central axis of the mound. The alignment is unlikely to be accidental, and whether or not it was intended as ritual, it gives a single morning each year a quality found nowhere else on the site.

The shrine at the front of the kofun, Higashinomiya-sha (東之宮社), still functions as a place of worship. Visitors are asked not to enter the shrine precincts uninvited or make noise nearby. The site has no lighting, no restrooms, and no shops; bring water and turn back well before dusk.

What Lies Around the Kofun

Inuyama is unusual in having both this early kofun and Aotsuka Kofun (青塚古墳), a slightly later keyhole-shaped tomb that is the second-largest in Aichi Prefecture. Aotsuka Kofun Historic Park, about 30 minutes away by car, includes a guidance facility displaying replicas of the Higashinomiya mirrors — a good complement for visitors who want to see the artifacts close up.

Inuyama Castle, perched on its own bluff above the Kiso River, holds the oldest surviving castle keep in Japan, dating to the late 16th century. The Old Town below the castle preserves a stretch of merchant houses and food stalls along a single main street. Naritasan Nagoya Betsuin, a branch of the famous temple in Chiba, occupies a nearby hill and offers sweeping views of the river. For those interested in tea culture, the garden at Urakuen contains the teahouse Jo-an, designated a National Treasure.

From Inuyama-yūen Station, a short walk down to the riverbank brings you to the launch point for the Japan Rhine boat cruises that thread through the gorge in warmer months. Cormorant fishing demonstrations run on summer evenings from a pier just below the castle.