Benthic foraminifera in brackish Lake Nakaumi were studied to detect the faunal changes due to human activities for several decades. Honj o area is located in the northwestern part of Lake Nakaumi, whose environment has been changed by the reclamation project started 25 years before.
　 We found that the critical faunal change have taken place owing to isolation from the main Lake Nakaumi area. Benthic foraminiferal occurrences are rare in this area. The original taxa characterized by Ammonia beccarii, Uvigerinella grabra, and Trochammina hadai along with elphidiids, miliolids become extinct or decreased their numbers after the isolation event (about 5.6-6.8 years). The main cause of such critical faunal change is due to the decrease in water salinity and the weakening of a halocline. The water circulation between the surface and the deep might have changed by the decreased salinity stratification, and resulted in the environmental change of the sediment/water interface.