We examine how Elinor Ostrom's eight design principles that characterize long-enduring, self-governed common-pool resource (CPR) institutions apply to Japanese irrigation CPR management. The eight design principles are "(1) clearly defined boundaries", "(2) proportional equivalence between benefits and costs", "(3) collective-choice arrangements", "(4) monitoring", "(5) graduated sanctions", "(6) conflict resolution mechanism", "(7) minimal recognition of rights to organize", and "(8) nested enterprises". These design principles refer to irrigation case studies in developing countries mainly and this has aroused our curiosity to examine them in Japan, which has a highly developed economy and where irrigators self-govern their irrigation CPRs (and where the irrigation institutions have been long-lasting and stable). The non-coercive strategic presence of an external entity (the central, prefectural and local government), although the external entity has a strong economy and has invested a lot, has significantly contributed to irrigators' self-governance of their CPRs in Japan. We find that non-coercive characteristic of the external entity, while the irrigators have strong endogenous institutional arrangements, has led us to moderate design principle seven to explain Japan's case. The design principles of monitoring (appropriators' behavior) and graduated sanctions are quite implicit rather than explicit - as Ostrom has generally described these two principles - in Japan's irrigation management. With all these, we have found that Ostrom's eight design principles are basic, well configured, and unique, and when we moderate principle seven, the eight design principles together can account for the success of Japan's long-enduring irrigation institutions that the irrigators formulate to self-govern their CPRs. (C) 2001 Elsevier Science B.V. All rights reserved.