Square mound (~40m per side) in Oshi, Asuka Village, just ~200m west of Kintetsu Asuka Station. Visitors can freely enter the superbly dressed cut-stone chamber (17.8m long), regarded as among the finest in Japan and the type-site of the "Iwayama-type chamber" used to date terminal-period tombs. Designated a National Historic Site in 1968.
Finds & research
Double-chambered corridor-type stone chamber (17.8m total, main chamber ~4.9m; dressed quartz diorite, opening south). The original coffin and grave goods were lost to looting and medieval grave intrusions; later-period haji ware, Sue ware, gaki ware, and old coins have been recovered from the chamber. The interior is open to visitors.
Two views on the occupant
Archaeological view
Square mound of ~40m per side (some sources ~45m), ~12m high, built in two tiers of rammed earth; one theory (Kunihiko Kawakami) reconstructs the upper tier as octagonal. Built in the early-to-mid 7th century. The double-chambered corridor-type stone chamber — 17.8m total, main chamber ~4.9m long — is constructed of precisely dressed quartz diorite ("Asuka stone"). Its craftsmanship defines the "Iwayama-type chamber," the type-standard for terminal-period cut-stone chambers (named by Taro Shiraishi), with parallels at Kotani and Minezuka Kofun. Occupant unknown; proposed candidates include Empress Saimei (Shiraishi), Princess Kibitsuhime, and Kose no Okara no Sukune.
Imperial Household Agency view
Not designated by the IHA. National Historic Site (designated 1968) managed by Asuka Village. Not an imperial or reference site. The stone chamber opens to the south and visitors may freely enter — a rare opportunity.
Field notes
The late kofun (6th–7th c.) of the Asuka area — Ishibutai, Miyakozuka, Takamatsuzuka, Kitora, Iwayama, Kengoshizuka — form a network of square and octagonal mounds that define the final chapter of kofun culture. All are accessible in a single day by rental bicycle from Kintetsu Asuka Station; Iwayama itself is a 2–5 min walk from the station.