The Sata-dao group of graves with burial mound, where the excavations were carried out, is located in the urban area on the east side of Shōbara city in the northern part of Hiroshima Prefecture. The group was built between the end of the last third of the Middle Yayoi period (1st century BC) and the first third of the Late Yayoi period (1st century AD).
A series of excavations yielded the following results: Sata-dao No. 1 and 2 are square-shaped ditch-enclosed slightly elevated burial precincts (hōkei shūkō bo); No. 3 is a grave with rectangular burial mound and four corner projections (yosumi tosshutsugata funkyū bo); No. 4 is also a grave with rectangular burial mound and four corner projections, but has subsequently been modified and altered into a square-shaped burial mound; No. 5 is a square-shaped ditch-enclosed slightly elevated burial precinct (hōkei shūkō bo), where on the inside of the ditch-enclosed space a grave pit was confirmed.
Towards the end of the last third of the Middle Yayoi period after repeated burials of several individuals, the burial mound of the Sata-dao No. 3 grave was completed. At grave No. 4, which is from the same phase, in the lower strata of the burial mound the surface of the excavation of a grave pit could be confirmed. In other words, it became clear, that at the graves No. 3 and 4 with rectangular burial mound and four corner projections, grave pits were first excavated before the burial mound was finally taking shape. Then the deceased persons were buried within and thereafter the grave pits were backfilled with the soil of the pit excavation. The so repeated iterations of grave pit excavation, burial and backfilling gradually produced the mound. However, on the other hand at Sata-dao grave No.1 and 2 the burials took place after the earthworks of the mound were nearly completed.
According to the results of the surveys, within the same group of graves one could confirm that the construction methods of the burial mounds changed: from the type, where the mound and the burial facilities are simultaneously constructed [co-occurrent progression type] of the end of the last third of the Middle Yayoi period to the type, where burial mound construction proceeds first [“mound first” type] of the first third of the Late Yayoi period. It became clear that is a rather rare group of graves with burial mound. This group is not only constructed during a relatively early stage of Yayoi period graves with burial mound, but also it became obvious that this is a very important site, if one is taking into consideration the development of construction methods of burial mounds of Yayoi period graves. Hereafter, for an understanding of the development of construction methods of burial mounds of Yayoi period graves the site is offering important insights.