The horizontal stone chamber of the back circular part was opened and known since early time, but by the excavation and research in 1958, it was discovered that there were also horizontal stone chambers in the back square part and the western narrow terrace.
後円部の横穴式石室は古くから開口しており存在が知られていたが、1958年の発掘調査で後方部と西側の造り出し部分にも横穴式石室があることが判明した。