Temporal species-diversity changes in Japanese Cretaceous inoceramid bivalves were analyzed from an extensive literature survey and statistical analysis, with the following results: (1) Species diversity increased gradually from the Upper Albian to Lower Campanian, and then dropped suddenly across the Lower/Upper Campanian (LCa/UCa) boundary; (2) There is no statistical correlation between ammonoid and inoceramid diversity changes in Japan, which must reflect the different ecologies of both groups; (3) Relatively high extinction ratios occurred at boundaries near Oceanic Anoxic Events (OAEs). The extinction events at the Albian/Cenomanian, Cenomanian/Turonian, and Turonian/Coniacian boundaries were potentially caused by OAE1d, 2 and the onset of OAE3, respectively. The drastic diversity decrease at the LCa/UCa boundary probably resulted from an abrupt and large-scale relative sea-level fall in the Yezo forearc basin; and (4) The pattern of diversity changes is similar to that of long-term (2nd-order) eustatic sea-level changes. The following hypotheses are presented as the cause of these phenomena: changes in shelf area, the primary inoceramid habitat, controlled their diversity, or changes in the Cretaceous outcrop area (rock volume) associated with sea-level changes controlled their diversity. It is possible that a combination of both factors controlled diversity patterns.