The is a group of six kofun burial mounds located in the Kaminakatsui neighborhood of the city of Maniwa, Okayama Prefecture, in the San'yō region of Japan. The tumuli were collectively designated a National Historic Site in 2008. The tumuli area considered to be representative of large chieftain tombs of the late Kofun period of the Kingdom of Kibi. Background The Ōya-Sada Kofun Cluster is located by the Nakatsui River, which flows through a basin surrounded by the Kibi Plateau, in southern Maniwa. A common feature is that all are -style rectangular burial mounds, with a stepped (pyramid) structure and a stone wall layer of outer stones. Ōya No.1 burial mound was built in the latter half of the 7th century. Archaeological excavations have unearthed gold-decorated iron swords and other items from the cut stone burial chamber. It is believed to date from the late 7th century. The Sada Kofun cluster is located at the foot of the mountain 1.5 km east of the Ōya No. 1 Kofun, and consist of five tumuli. The Sada-Higashi and Sada-Nishi tumuli were once regarded as a single "two conjoined rectangles" type (zenpō-kōhō-fun (前方後方墳)); however, subsequent research indicated that they are separate tumuli. Sada-Higashi Kofun The Sada-Higashi Kofun is rectangular, measuring 25 meters north to south, 18 meters east to west, and 6 meters high. The mound is surrounded by two tiers of stones. Excavations were conducted from 1994 to 1997. The burial facility is a horizontal stone chamber orientated to the south. The total length of the burial chamber is 11.6 meters including the entry passage. Inside were four Haji-style ceramic coffins (2 tortoise shell-shaped, 2 gabled house-shaped, 1 four-pillar house-shaped) and a nailed wooden coffin. Grave goods included cearrings, small beads, gold thread iron long swords, arrowheads, halberds, and metal fittings, horse harness and metal fittings, agricultural tools and Sue ware pottery (bowls, lids, vases, jars). It is estimated to have been built around the end of the Kofun period, around the first half of the 7th century, and is the oldest among the cluster of kofun. Sada-Nishi Kofu The Sada-Nishi Kofun was built adjacent to the west side of the Sada-Higashi Kofun on the southern foot of a hill overhanging the east bank of the Nakatsui River. The mound is rectangular, measuring 16 meters north to south, 14 meters east to west, and 4.5 meters high and is surrounded by two