Takeshi KAGAMIYAMA estimated that kogo-ishi were built after the middle of the seventh century because the gap between the holes for posts in front of the arranged stones was about three meters and nearly equal to 2.95 meters) using the "toshaku" (unit of measurement during the period of Tang Dynasty in China) which was used from the middle of the seventh century.
鏡山猛は、列石前面の掘立柱穴の間隔が約3mで並んでおり、唐尺（一尺＝29.5cm）の十尺（2.95m）とほぼ等しいことから唐尺が使われた7世紀中頃以降の築造とした。