Geochemical compositions of sediments from the tidal flat in Kasaoka Bay and those of the Tamashima and Ashida-gawa areas of the neighboring bays in Seto Inland Sea were examined. The Kasaoka bay is one of the natural coasts remained within this inland sea and is well known for Horseshoe crab habitat at present. Heavy metals are useful elements for evaluation of coastal environments in comparison to iron concentration. A comparative study of other data from the estuaries of big cities in Seto Inland Sea reveals that zinc concentrations from Kasaoka, Tamashima and Ashida-gawa samples show similar level to those of the big cities (Urban trend). However, some of the Kasaoka samples show the lowered Zn concentration compared to the Urban trend. This group plots intermediate of the Urban trend and the normal sediment trend. Decreased concentration of Zn may be related to seawater circulation and probable restoration of natural ecosystem in Kasaoka Bay.