An interesting occurrence of large fossilised corbiculas(Corbicula japonica, which lives in brackish water), coexisting with living specmens, has been known by fishermen at a transitional area between shelf and slope in the northern central part of Lake Shinji at a depth of about 3 m. The possibility of the occurence being an ancient shell mound is low, as no man-made remains have been dredged in the present area. The fossilified corbicula give a ^14C age of 1926＋124-122 yrs B.P. This age suggests that the corbiculas flourished to form the corbicula bed at the late Holocene regression stage, as Lake Shinji is known changed from a marine embayment in the Jomon Age to a brackish coastal lagoon in the Yayoi Age. Echo-sounding surveys in the northern central part of Lake Shinji show that a reflector exists less than 1.5 metre below the bottom and this is traceable to the subaqueous exposure of the corbicula bed. The reflector could be penetrated by a newly-devised gravity corer, and it was found that the reflector definitely consists of corbiculas. It is inferred that Corbicula japonica flourished in great numbers in accordance with a drastic environmental change in Lake Shinji, from the embayment of Jomon Age to the brackish lake of Yayoi age.