The Ultra-Tamba Terrane within the Inner Zone of Southwest Japan is interpreted as a Guadalupian (Middle Permian) to Lopingian (Late Permian) subduction-related accretionary complex. The Kozuki Formation is the UT3 unit of the terrane and is mainly present in eastern Okayama and western Hyogo prefectures. Recently, it was shown that the Kozuki Formation is also distributed in the Kanbayashi area of northern Kyoto Prefecture based on the similarity of its lithological features and tectonic position. The exact age of the formation was unknown until now. In this study, radiolarian fossils are discovered in two sections, allowing for the determination of the ages of cherts and clastics within the Kozuki Formation. The cherts yielded radiolarians such as Curvalbailella u-forma (Holdsworth and Jones) and Curvalbailella chilensis (Ling and Forsythe), which indicate a latest Carboniferous or early Cisuralian, Early Permian age, and the mudstones yielded radiolarians such as Latentifistula banchengensis Wang, Pseudotormentus cf. kamigoriensis De Wever and Caridroit, Quadriremis gracilis (De Wever and Caridroit), which indicate Guadalupian to Lopingian, but most likely Guadalupian, age. This study establishes the Kozuki Formation in the studied area as a part of the UT3 unit of the Ultra-Tamba Terrane, confirms its age correlation with other regions, and emphasizes the northeasternmost extension of the unit.