We found a mylonite zone up to 1300 m in width in a granodioritic pluton in Nanyo City, Yamagata Prefecture, and named it the Ringou Mylonite Zone. Geological and petrographical features of the mylonite zone, situated between the Tanagura Tectonic Line in the south and the Oisawa Tectonic Zone in the north, are described. The mylonite zone shows sinistral shear sense, which is in harmony with that of the Tanagura Tectonic Line in the southern Abukuma Mountains. A K-Ar hornblende age of granodiorite (114 Ma) distributed about 5 km east of the mylonite zone is similar to the isotopic ages obtained from Abukuma Granites. On the other hand, granodiorites on the western side of the mylonite zone are petrographically identical to some of the granitic rocks in the Asahi Mountains. From these facts, we infer that the Ringou Mylonite Zone corresponds to the northern extension of the Tanagura Tectonic Line.