Fossil diatom assemblages were investigated from the Onbara and Tatsumi-toge areas in the Ningyo-toge Formation, which is distributed around the border of Okayama and Tottori Prefectures and is estimated to be Late Miocene to Pliocene in age. Plant fossils have also been reported from these areas. In total five samples, three from Onbara and two from Tatsumi-toge, were observed in order to examine the relationship between the diatoms at the two sites. The diatom assemblages were all dominated by freshwater centric diatoms with a relative frequency of 97–100%. Two samples from the Onbara area, original materials of fossil plants, and one sample from the Tatsumi-toge area, near the horizon of plant fossil sampling site, were dominated by
Cyclotella kohsakaensis H.Tanaka & H.Kobayasi and
Aulacoseira sp., while one sample taken near the plant fossil horizon in Onbara area and one from Tatsumi-toge, the horizon of the plant fossil sample site, were dominated by
Cyclotella iris Brun & Hérib. s.l. and
Aulacoseira ambigua （Grunow） Simonsen, indicating each area consists of at least two horizons. These species compositions do not correspond to that of Naru, a vicinal site whose diatom species composition was previously reported, suggesting it also belongs to a different horizon.
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