Examination of tidal flat environment of the Kasaoka Bay in Seto Inland Sea was carried out from geochemical analysis of suspended solid (SS) in relation to tidal variation. Geochemical compositions of Zostera marina (submerged plan) was also examined for evaluation of water environment. Material filtered on quartz filter was observed using SEM and further analyzed by XRF to evaluated geochemical compositions. This method is a new examination for the SS in view point of both constituent observation and geochemical compositions. SS value varies with tidal change, namely, it increases toward the ebb tide. This is suggestive of increasing of detritus of tidal flat environment. SEM observation demonstrates that the material of the spring tide was composed of diatom mainly of Skeletonema sp. and some Chaetoceros sp. and Asterionella sp. were observed. The SS of ebb tide was fine grained detritus including fragment of biogenic material. Geochemical compositions of the SS of spring is characterized by higher contents of P_2O_5, while that of ebb tide is enriched in heavy metals such as Zn, Cu, Cr, Fe and Mn. Although Br, F and Cl were measured for SS, iodine was not detected for all SS samples. This is suggestive of lacking of intake of iodine for diatom of these samples. Geochemical composition of Z. marina showed higher concentration of Zn, Cu, TS (total sulfur), iodine, MnO and P_2O_5. Comparative examination of Z. marina from other localities of Seto Inland Sea shows that the Kasaoka Bay samples show higher values in Zn, Cr, MnO and P_2O_5, suggesting enriched water environment of antholopologenic inputs. The Kasaoka Bay samples, however, show similar values to those of the Ise-Matsusaka area of Pacific Sea, excluding Zn concentration. I- P_2O_5 diagram is useful for evaluation of nutrient competition between Z. marina and marine plankton and eutrophication of marine condition.