A nepticulid leaf-mine ichnofossil, Stigmellites janggi Sohn and Nam, n. ichnosp., is described on the basis of a fossil leaf of Fagus from the early Miocene Geumgwangdong Formation in Pohang Basin. This mine trace is characterized by a linear-blotch type with a clear centric frass trail composed of closely and randomly dispersed pellets filling the mine width in the early stage. We found traces of a possible egg case and an exit slit from the leaf. These features are most consistent with those produced by members of Nepticulidae. Our record demonstrates for the first time the trophic association of Nepticulidae with Fagus in the Miocene and suggests the persistence of a long-term association between the insect family and the plant genus from the Miocene to the present. Other Nepticulidae leaf mines in the Miocene and leaf-mine fossils from the Geumgwangdong Formation are briefly reviewed.