不動山古墳

The mound lies in the south of Gunma Prefecture, on the right bank of the lower Inogawa river. Four keyhole-shaped mounds are found in the area (Watanuki Kannonyama, Fudōyama, Fugenji-ura and Iwahana Futagoyama, the last of which has disappeared), together with a number of round mounds; these make up the Watanuki group.

The front part, which faces west-southwest, has been heavily levelled, and an image of Fudō is enshrined on the summit. Fukiishi (cobbles) and haniwa have been found on the outer surface. A projection (tsukuridashi) on the north side of the constriction has yielded a row of haniwa and Haji ware, including Izumi-type high-footed cups. A double moat surrounds the mound. The burial facility is not clear, but the body of a large boat-shaped coffin hollowed from tuff remains on the summit; its lid has been lost.

The mound measures 94 metres in length. The rear circle is 54 metres in diameter, 18 metres across the summit and 10.1 metres high. The front part is 55 metres wide and its height is estimated at 9.1 metres. The constriction is 36.5 metres wide.

The Watanuki group is thought to have been built in the order Fugenji-ura, Fudōyama, Iwahana Futagoyama and Watanuki Kannonyama. This suggests that powerful local lineages existed in the area in the 5th and 6th centuries.