In the Shimoina area of southern Nagano prefecture is a group of ancient burial mounds distributed on river terraces of the right bank of the Tenryū River. There were once more than 520 kofun in Iida, of which a total of 22 are collectively called the "Iida Kofun cluster". These tumuli consist of 18 keyhole-shaped () and four scallop-shaped tumuli. The presence of so many of this style of tumuli point to a very strong political and cultural connection to the ancient Yamato kingdom. The Iida tumulus group is presumed to have been built in the middle-late Kofun period in the latter half of the 5th century and the end of the 6th century. During this period, it is postulated that the Iida area was a major horse breeding base for the Yamato kingdom, and the introduction of horses led to the development of inland transportation networks and inter-regional trade, which is reflected in various aspects of each tumulus. Of the group, 11 keyhole-shaped and two scallop-shaped tumulus received National Historic Site designation in 2016.
