古墳時代初期（３世紀ごろ）につくられた、福岡平野で最も古い前方後円墳です。後世の改変によって築造当初の形は留めていませんが、後円部は那珂八幡宮が鎮座することから、比較的よく残っています。

　社殿横の発掘調査では、三角縁神獣鏡（さんかくぶちしんじゅうきょう）や勾玉（まがたま）などが副葬された木棺墓（もっかんぼ）が見つかり、奴国（なこく）の有力者が葬られたと考えられています。この地域の古墳時代の始まりを考える上で非常に重要な遺跡であり、市の史跡に指定されています。

A keyhole-shaped burial tomb (kofun) that forms part of the Imajuku Kofun Cluster, this is the oldest such tumulus in the Fukuoka Plain, built in the early Kofun period (around the 3rd century). Due to alterations in later times, it no longer retains its original form; however, the rear circular portion is comparatively well preserved, as Naka Hachimangu Shrine stands upon it.

An excavation conducted beside the shrine buildings uncovered a wooden-coffin grave containing grave goods such as a triangular-rimmed mirror with divine-beast motifs (sankakubuchi shinjūkyō) and magatama beads. It is thought that a powerful leader of the ancient polity of Na (Na no Kuni) was interred here. This site is extremely important for understanding the beginnings of the Kofun period in this region and has been designated a municipal historic site.