This paper aims to analyse and elucidate key conditions for the sustainability of community-based fisheries management (CBFM) on unrestricted fisheries in Japan. It re-examines more than 50 years’ worth of empirical evidence regarding the outcomes of the socio-economic rationality of the self-imposed management of common property, with particular reference to splendid alfonsino (Beryx splendens) fishing grounds off Katsuura, Chiba Prefecture, and the Small-Type Coastal Fishing Vessel Fishery Cooperative (STCC) there. It shows that the exclusive use of fishing grounds and other factors, including communal solidarity and a social milieu advocating fishermen’s protection of fishing grounds observed roughly 20 years ago, have for more than 50 years continuously existed as conditions necessary to establishing sustainable CBFM in unrestricted alfonsino fisheries using waters off Katsuura. At present, however, it should be recognised that the resource management of alfonsino as a migratory species would be required with different types of fishery operations in fishing grounds other than STCC’s operation near Katsuura. To resolve this issue, a co-management system involving a resource management plan and fisheries risk management plan under the fraternal insurance of the fishery was recently introduced in collaboration with the Japanese government, Chiba’s prefectural government, and the STCC. As a type of adaptive fisheries management system in Japan, this co-management system would play an important role in securing key sustainable conditions for CBFM in unrestricted, self-managing fisheries in alfonsino fishing grounds off Katsuura, Japan.