Tamaoka Kofun Cluster, Kofun burial mound cluster in Kasai, Japan

The Tamaoka Kofun Cluster is a group of seven burial mounds on a terrace on the east bank of the Kakogawa River in Kasai, Hyogo Prefecture. Its main tumulus, the Tamaoka Kofun, is a keyhole-shaped mound 109 meters long, built in three tiers and ringed by a 20-meter moat; a hole in its rear circle exposes the remains of a tuff sarcophagus. The cluster also includes the 35-meter Kuwanzuka Kofun, the 17-meter Dantōzuka Kofun, two Baizuka tumuli, the Shinozuka Kofun and the relocated Aizen Kofun.

The Tamaoka Kofun was designated a National Historic Site in 1943; the other tumuli were added in 1978, and the protected area was expanded in 1997. The cluster once counted over 30 tumuli, most destroyed by residential development. The main mound dates from the middle Kofun period, and its burial chamber was robbed in antiquity, leaving a hole in the rear mound and traces of a tuff sarcophagus.

The name Nichijō-hime, from the Harima Kokudoki tale of her meeting with two princes, is still used by Kasai city in its tourism promotion. Haniwa recovered from the mounds include house-shaped and chicken-shaped examples, and the Kuwanzuka tomb yielded swords, armour, hoes and clay objects such as pestles and disc shapes.

The site is reachable on foot in about 20 minutes from Hōjōmachi Station on the Hōjō Railway Line. It is best to visit early in the day on warm days, as there is little shade on the terrace.

The Harima Kokudoki, a Heian-period gazetteer, records the tale of Nichijō-hime, whose name began with a meeting with two princes at this mound. Kasai still uses her name in tourism promotion, and the story is set in the Kamo district entry of the text.