Minotani Kofun Group

Mitani Tumuli — A Quiet Valley That Holds the Oldest Dated Kanji in Hyogo

Tucked into a small valley in the mountains of northern Hyogo, the Mitani Tumuli (Mitani Kofun-gun) are a group of four reconstructed burial mounds from the 7th century. They sit within the leafy grounds of Tsurugigaoka Park in Yabu City, surrounded by a quiet copse of broadleaf trees and broad expanses of lawn. To casual visitors the site may look simply like a peaceful country park — but archaeologists know it as one of the most important late Kofun-period sites in the Tajima region. From the largest mound came an iron sword inscribed with a date corresponding to the year 608 AD, the oldest dated kanji inscription ever found in Hyogo Prefecture.

This article introduces the history of the site, the reasons it was designated as a National Historic Site, what visitors can see today, and how to enjoy a visit to this evocative corner of ancient Japan.

Historical Background

The Mitani Tumuli are located on the left bank of the lower Yagi River, a tributary of the Maruyama River, in a small side valley known locally as "Mitani" (literally "Winnowing-Basket Valley"). The mounds were first revealed in 1983, when the Yoka Town Board of Education conducted a rescue excavation in advance of the construction of a new sports complex called Tsurugigaoka Park. What started as a routine pre-construction survey turned into a discovery of national significance once an inscribed iron sword was unearthed from the largest mound.

Originally five mounds were identified at the site, numbered 1 through 5. Four of these — Tumuli No. 2 to No. 5 — have been preserved and reconstructed in their original locations. The mounds were built in sequence from No. 2 onward, becoming progressively smaller and positioned slightly higher on the valley slope as time went on. Based on the pottery recovered, archaeologists place their construction in the early to mid-7th century. This was the era of Empress Suiko and Prince Shōtoku in the Asuka region near modern Nara — a formative chapter in the early Japanese state.

About four kilometres to the southeast lies the larger Ōyabu Tumulus Group, thought to be the burial place of leaders who governed Tajima as kuni-no-miyatsuko (regional chieftains). The people interred at Mitani are believed to have been a supporting clan that helped these chieftains administer the region.

Why the Site Was Designated a National Historic Site

The Mitani Tumuli were designated a National Historic Site on 18 December 1992, with an area of 7,000 square metres protected. Two factors drove the designation: the rarity and importance of the inscribed iron sword from Tumulus No. 2, and the unusual architectural features of the mounds themselves, which together preserve a vivid record of cross-cultural exchange between the Japanese archipelago, the Korean Peninsula, and the Asian continent in the 7th century.

The Inscribed Iron Sword of 608 AD

The most famous discovery from Mitani is an iron sword found among the grave goods of Tumulus No. 2. Near the base of its blade, silver-inlaid characters spell out "戊辰年五月" — "the fifth month of the year of the elder earth dragon (Boshin)." Most scholars identify this Boshin year with 608 AD, although a minority view dates it to 668 AD. Either way, this is the oldest dated kanji inscription so far recovered in Hyogo Prefecture, and only the second iron sword in Japan known to bear an inscription including a sexagenary cycle year.

In 1996, the 103 items excavated from the chamber of Tumulus No. 2 — including the inscribed sword, weapons, horse trappings, gilt-bronze earrings, and pottery — were collectively designated a National Important Cultural Property. The original objects are conserved off-site, with carefully made replicas displayed inside the reconstructed tomb chamber.

Korean-Influenced Architecture in Tumulus No. 3

Tumulus No. 3 is a roughly 17-metre circular mound built in three tiers, each ringed by a stone alignment. This triple ring of stones is unusual even within Tajima, where stone-aligned tombs are otherwise relatively common, and is rare across Hyogo as a whole. Comparable structures are known from the Korean Peninsula — for example the famous Changnyeong Gyodong Tumulus No. 4 near Busan — leading some specialists to argue that the building technique reached Tajima from the Korean kingdoms.

Highlights for Visitors

The Park Itself

The Mitani Tumulus Park was completed in May 1994 as a heritage preservation project under the Agency for Cultural Affairs. The four mounds are arranged within a sheltered hollow, ringed by a screen of mixed woodland that gently separates the site from the adjacent sports facilities. Between and around the mounds there are open lawns, with simple granite blocks placed as benches. The atmosphere is restful and contemplative — well suited to walking, sketching, or quiet observation.

Tumulus No. 2 and Its Stone Chamber

Tumulus No. 2, the largest of the four mounds, is a circular burial measuring about 14 metres across and standing 3.5 metres high after restoration. Its corridor-style stone chamber (yokoanashiki sekishitsu) is 8.6 metres long, 1.2 metres wide, and 1.7 metres high, of the "sleeveless" (musode) type, in which there is no architectural distinction between the burial chamber and the corridor. The walls are built up with carefully fitted rectangular stones — a tidy and stable construction — and the floor is paved with small pebbles.

One ceiling stone has been replaced with specially reinforced tempered glass, allowing daylight to filter into the chamber. Visitors can therefore peer in to see a thoughtful reconstruction of the burial as it would have appeared at interment: a board representing the deceased lies in the centre, with replica pottery, an iron sword, and arrows arranged around it. The black-lacquered sheath of the inscribed sword glints quietly in the diffused light.

The Replica of the Inscribed Sword

The replica of the dated sword is one of the most striking objects on display. The blade is roughly 65 centimetres long, the sheath 70 centimetres, and the hilt 15 centimetres. The sheath is finished in black lacquer and fitted with gilt-bronze mounts, while the hilt is wrapped in silver wire with finely cut grooves. The form belongs to a class known as keitōtachi ("rounded-pommel sword"), examples of which also survive in the Shōsōin treasure house at Nara — almost certainly Asuka-region work brought to Tajima as a prestige gift.

Geomantic Siting

Standing in the centre of the group, visitors may notice that the mountain slopes pinch inward to form a sheltered pocket behind the mounds to the north, while the land opens out southward to lower terrain. The chamber entrances all face south. This careful match between landscape and orientation reflects fūsui (Chinese-style geomancy) — the same principles guiding the layout of contemporary tombs in the Asuka region near Nara.

Visitor Information

The site is freely accessible as part of Tsurugigaoka Park, which also includes a gymnasium and indoor pool. The kofun park area is laid out for self-guided strolling, with explanatory signs. Comfortable walking shoes are recommended, as the lawns and paths are uneven in places.

• Admission: free
• Open: outdoor site, accessible year-round
• Best seasons: late spring for fresh greenery, autumn for foliage
• Time required: 30–60 minutes

Around the Site

Yabu City offers a rewarding range of cultural and natural attractions for travellers extending their visit beyond the Mitani Tumuli.

Ōyabu Tumulus Group

About four kilometres to the southeast lie the four large Ōyabu tumuli, thought to be the resting places of Tajima's ruling chieftains. Their massive corridor-style stone chambers are among the most impressive of their type in the Tajima, Tango, Tanba, and Harima regions combined.

Yagi Castle Ruins

Also designated a National Historic Site, Yagi Castle is a large mountain castle complex once held by the Yagi clan during the Sengoku period and later by Hashiba Hideyoshi's general Bessho. Hiking up to the ruins offers panoramic views over the Tajima valleys.

Yabu Heritage Road

The city has organised many of its cultural sites along a "Yabu Historical Road" itinerary that links shrines, temples, traditional villages, and archaeological sites. The local tourism office can recommend a route to suit your time and interests.

Takeda Castle Ruins

A short drive away in neighbouring Asago City, the celebrated Takeda Castle Ruins — known as the "Castle in the Sky" for the morning seas of cloud that wrap its stone walls in autumn — make a stunning addition to a Tajima itinerary.