Devonian chert beds in the Permian Kozuki Formation of the Ultra-Tamba Zone are reported from Mimasaka City, Okayama Prefecture, Southwest Japan. The Kozuki Formation consists mainly of Permian clastic rocks and felsic tuffs with basalts, Carboniferous limestone, and Early Permian cherts, and is therefore interpreted as a Permian accretionary complex similar to the other formations of the Ultra-Tamba Zone. The Devonian chert beds are frequently intercalated with black layers composed of magnetite, in which a well-preserved radiolarian fauna occurs. These magnetite beds with radiolarian shells indicate the sedimentary origin derived from hydrothermal activity on the seafloor. The radiolarian fauna contains Holoeciscus foremanae and H. elongatus, as well as Ceratoikiscum avimexpectans and Archocyrtium sp., indicating the Fammenian Age of Late Devonian. These are the oldest fossils in the Chugoku District and the chert beds represent one of the oldest allochthons within the accretionary complexes of Japan, similar in age to the Devonian chert of the Nedamo Belt in the Tohoku District.