The distribution and population density of the brackish-water bivalve, Corbicula japonica, in the coastal waters of Lake Shinji were summarized from the literature. It is thought that the primary habitat of C.japonica was limited to the vicinity of the Oohashi River before the early Showa era. Population density was relatively low, slightly less than 1,000inds./m^2 from 1930 to 1960. After that, the population density of C. japonica began to increase starting in the mid-1960 s and rose to its highest level-10,000～20,000 inds./m^2-in the 1980 s. The increase of C.japonica coincides with a significant increase in chl-a, implying the development of better food resources for C. japonica. On the other hand, the area inhabited narrowed to the lake shore shelf. Although a large die-off of C. japonica occurred in the autumn of 1997, the population density of C. japonica has recovered rapidly, i.e. the population has maintained a density of 5,000-10,000 inds./m^2 since 1999.