Yes, curious. But almost unknown even to Japanese people, I happened to find its existence in Yoshonogawa City's municipal website last year.
This round tumulus is supposed to be built between end of 6th century AD and early 7th century AD, the final stage of the tumuli era which began mid 3rd century AD. The only similar example that I know is Yama-no-Ue Kofun tumulus in Gumma Prefecture which has stone epitaph beside its mound.
The basin of Yoshino-gawa river which flows below this tumulus is a source of green schist and also the origin of the Imbe tribe who administered ritual celemonies in those days. So this area is the most menhir-concentrated land in Japan.
At last, the Imbes seemed to merge their burial mound and sacred stone.
Something is not right. This message is just to keep things from messing up down the road