In Tsuyazah tidal nat in North Kyushu, the sediment from the surface to the depth of 50cm were collected at 10cm intervals. Shell size of Umbonium moniligerum, Batilailia zonalis and Macoma ingongrua, which were included in the sediment, were measured. The dead shells were not greatly different from primary shell size-frequency distribution at the surface, at a center of their habitation. This tendency was found in the dead shell at other depth also. On the other hand, outside the habitat, the shell size distribution histogram of B. zonalis showed right backing away. At other depth, there was a little number of shells outside their habitat. Therefore, histogram of dead shell size distribution does not show a tendency.