The Site of Japan's First Excavation by Japanese Scholars
The Okadaira Shell Mound (陸平貝塚, Okadaira Kaizuka) is a Jomon-period shell mound on a plateau on the south shore of Lake Kasumigaura in Miho Village, Inashiki District, Ibaraki Prefecture, which in 1998 was designated a Historic Site.
The Ibaraki Prefectural Board of Education gives its location as 391-2 Kaiuri, Mamiyama, Miho, and adjoining lots, with a designated area of 65,192.34 square metres managed by Miho Village.
In 1879 it was investigated by Sasaki Chujiro and Iijima Isao, then students at the University of Tokyo, and it is remembered as the site of the first scientific investigation carried out by Japanese researchers.
Eight Mounds Ringing a Plateau
According to the board of education, the shell mound lies on an isolated plateau 20 to 30 metres above sea level on the south shore of Kasumigaura. Surveys by the Okadaira research group formed in 1987 confirmed eight mounds ringing a plateau about 250 metres east to west and 150 metres north to south, forming a ring-shaped shell mound.
Miho Village describes the plateau, the Annaka plateau, as probably an island in Kasumigaura at the time, and notes that the valleys and other natural landforms where the Jomon people lived survive almost intact around the mounds.
On the period of formation, the prefecture says Initial to Final Jomon, while the village says Initial (about 7,000 years ago) to Late Jomon (about 3,500 years ago), so the end date is expressed differently.
What the Finds Reveal
The board of education lists shells, chiefly clams with other bivalves, fish bones including black sea bream and sea bass, and animal bones including wild boar and deer.
Tools besides pottery include stone querns, arrowheads, polished axes, bone and antler implements and net sinkers, showing a diet drawn from both sea and hills. Pit-dwelling remains on the flat top of the plateau have revealed where people lived.
The prefecture ranks the Okadaira Shell Mound as the representative of some 180 shell mounds around Kasumigaura, for the length of time it was formed and for a scale that covers the whole plateau.
Visiting the Okadaira Shell Mound
The Historic Site is laid out as Okadaira Shell Mound Park, with the Miho Village Cultural Properties Centre inside it. Ibaraki's official tourism site says the centre displays pottery excavated at the mound and is open from 9:00 a.m. to 5:00 p.m., closed on Mondays and public holidays (and the following Tuesday when a Monday is a holiday).
Miho Village notes that the site and park are governed by a management ordinance, and uses other than visiting require prior application. It is about 40 minutes by car along Route 125 from the Sakura-Tsuchiura interchange of the Joban Expressway, and there is parking. Enquiries: Miho Village Cultural Properties Centre, 2359 Tsuchiura, Miho; telephone 029-886-0291.
Q&A
- When was the Okadaira Shell Mound designated?
- It was designated a Historic Site on 11 September 1998.
- How is 陸平貝塚 read?
- Okadaira Kaizuka.
- Why is the Okadaira Shell Mound famous?
- In 1879 two University of Tokyo students, Sasaki Chujiro and Iijima Isao, investigated it, the first scientific investigation by Japanese researchers.
- What shape is the Okadaira Shell Mound?
- A ring of eight shell mounds surrounding a plateau.
- Is there a museum at the Okadaira Shell Mound?
- Yes; the Miho Village Cultural Properties Centre in the park displays finds, open 9:00 a.m. to 5:00 p.m., closed Mondays and holidays.
Basic Information
| Name | Okadaira Shell Mound (陸平貝塚) |
|---|---|
| Location | 391-2 Kaiuri, Mamiyama, Miho Village, Inashiki District, Ibaraki Prefecture, and adjoining lots |
| Designation | Historic Site |
| Year Designated | Designated 1998/9/11 (Heisei 10) |
| Quantity | Not recorded in the Agency for Cultural Affairs database |
| Dimensions | Designated area 65,192.34 square metres |
| Owner | Manager: Miho Village |
| Public Access | Open as Okadaira Shell Mound Park; centre closed Mondays and holidays |
References
- Ibaraki Prefectural Board of Education: Okadaira Shell Mound (Japanese)
- https://kyoiku.pref.ibaraki.jp/bunkazai/kuni-110/
- Miho Village: Okadaira Shell Mound Park (Japanese)
- https://www.vill.miho.lg.jp/kankou-event/minohobunkazai/okadairakaitsuka/page000466.html
- Ibaraki Tourism: Miho Village Cultural Properties Centre (Japanese)
- https://www.ibarakiguide.jp/spot.php?mode=detail&code=1382
Last verified: 2026.09.27