The parasitoid assemblage associated with a lepidopteran leafminer, Coptotriche japoniella (Tischeriidae), on an evergreen tree, Eurya japonica (Theaceae), was studied in the center of Japan to explore parasitoid coexistence mechanisms. The leafminer supported 12 parasitoid species. Eight abundant or common species were classified into five guilds according to their koinobiont/idiobiont mode and host-instar utilization pattern: early larval koinobiont, mid-larval idiobiont, mid-larval–late larval idiobiont, late larval–pupal idiobiont, and pupal idiobiont. The early larval koinobiont (Orgilus kumatai) and mid-larval idiobiont (Achrysocharoides sp.) seemed to be specialized on the host, whereas the members of the other guilds had a wide host range. The mid-larval–late larval (Cirrospilus diallus and Pnigalio sp.) or late larval–pupal idiobionts (Chrysocharis albipes, Apleurotropis kumatai, and Pleurotroppopsis japonica) facultatively hyperparasitized half of spinning larvae or pupae of the early larval koinobiont. These results suggest that parasitoid coexistence in this assemblage is greatly promoted by high levels of facultative hyperparasitism by idiobionts with wide host ranges on the dominant koinobiont.