Kochi Prefectural Museum of History ／ TEL：088-862-2211

Oko Castle Ruins are located in Okoyama, a mountain in Oko-cho Yahata in Nangoku City. It is one of the representative castle ruins in Kochi Prefecture and part of it was designated as a national historical site, “The Oko Castle Ruins” on July 28, 2008. These are the ruins of the residential castle of Chosokabe, a military commander on Shikoku Island in the period of Warring States.

There are several hillsides on the Kacho Plain. The serpentine Mt Oko (mountain) is one of them. 97m in altitude, the mountain is steep in its east and south sides. The north side is slightly less hilly and west side slopes in a continuous grade to 49m. The Kokubu River runs at the foot of the south side, and the west and south sides are swampy. It seems that the swamp was there at the time the castle was built, offering natural protection. It seems that the north side was connected to the Shikoku Mountain district by a low ridge, but it is now separated by a prefectural road (old national road). So, with the construction of roads toward the top of the mountain to construct a park, the castle ruins were forced to change.

The excavation of the Oko Castle ruins was conducted as a part of surrounding environment adjustment, along with construction of the Kochi Prefectural Museum of History. In an earlier confirmation survey for prefectural ruins adjustment, the first to fifth excavation study was conducted from 1985 to 1989 (government subsidy) and the sixth excavation study was conducted in 1990 by the Kochi Prefecture Education Board.

Research was conducted at the central castle areas, with the secondary, tertiary and fourth concentric areas. At the central area, paved stone remains and building’s cornerstone ruins and remains of a land blessing event were found. In the tertiary area, earthwork/stone bonds, cornerstone building ruins and stair-like remains were confirmed. Additionally, a lot of Haji pottery and imported ceramics, as well as kawara roof tiles with engraved era names were found.

The excavation was carefully conducted so the current condition of the castle ruins would not change while cutting down and planting of trees. The unearthed condition was restored by filling the remains with earth.
It is unknown when Oko castle was built. Historians think the castle existed until around 1588, when Motochika Chosokabe moved to Otakasa (or Otakasaka) Castle. Its history includes the latter 15th century and the time around 1508 to 1509, when it fell to the enemy (during the time of Motohide Chosokabe).