Lake Nakaumi is a brackish estuary showing the high to medium range of salinity. Benthic animals resisting to anoxic environments are distributed in the deep bottom of Lake Nakaumi. In contrast, surface water is good condition for normal brackish animals. To clarify the capacity of carbonate shell animals for distribution in the Lake Nakaumi, the vertical distribution of carbonate shell animals on the artificial columns was investigated. On the basis of this result, it was considered the controlling factors and spreading mechanism of the carbonate shell animals such as Mollusca, Ostracoda and Foraminifera. They are summarized as following: 1) The carbonate shell animals inhabit to shallower zone than the lower limit of fluctuation zone of halocline (around 4.5 m depth) in the central Lake Nakaumi. 2) Many species are adapted to a wide range of salinity (at least 10-30psu). Species adapted to low salinity distribute around the mouth of Ohashi River. 3) The meiofauna are able to inhabit after the formation of the macro-faunal colonies. 4) Although the meiobenthos living to columns spread the distribution area from bottom, they can not distribute over the halocline. 5) The species distributed to upper part of fluctuation zone of halocline may be carried by bottom current from the lower reaches during the larval stage. 6) The species distributed to upper column may be carried by surface current during the larval stage. On the basis of the control factors and spreading mechanism, main species of Mollusca, Ostracoda and Foraminifera are illustrated the capacity for distribution in the Lake Nakaumi. Musculista senhousia, having a wide capacity, can inhabit everywhere except for bottom water zone showing anoxic condition.