Keyhole-shaped burial mounds first appeared in the latter half of the 3rd century mainly in the Kinki and Setonai regions, and became distributed from southern Tohoku (present day Miyagi Prefecture) to southern Kyushu (present day Kagoshima Prefecture).
　170 keyhole-shaped burial mounds have been confirmed in Miyazaki Prefecture. However, keyhole-shaped burial mounds are not found in all areas of the prefecture; rather, they are concentrated mainly in coastal areas facing the Sea of Hyuga, and inland are limited only to the north of the Miyakonojo Basin. Keyhole-shaped burial mounds are distributed among the current municipalities of Nobeoka City, Hyuga City, Kawaminami Town, Takanabe Town, Shintomi Town, Miyazaki City, Kunitomi Town, Nichinan City, Kushima City, and Miyakonojo City.
　The largest keyhole-shaped burial mounds include Saitobaru Mound No. 81 (Saitobaru City), Shimoyashiki Kofun (Shintomi Town), and Aoki Mound No. 1 (Miyazaki City). Evidence suggests that construction commenced mainly in plain areas of Miyazaki at the end of the 3rd century/beginning of the 4th century (Early Kofun period). From the middle of the 4th century to the beginning of the 4th century (latter half of the Early Kofun period), a form of keyhole-shaped burial mound unique to southern Kyushu emerged in major kofun groups in plain areas of Miyazaki, such as Ikime, Saitobaru, Mochida and Kawaminami, as well as the Osumi region. Known as “ekagamigata (hand mirror-shaped) keyhole-shaped burial mounds,” kofun of this type feature an elongated frontal section with a planar form shaped like a hand mirror.