The Seto Inland Sea is characterized by the semi-closed marine environment, composed of several seas called Nada or Wan. The tidal variation can be over 3 m during spring tide. The tide movement is significant for exchange and circulation of the material in the marine conditions. This study focused on an evaluation of marine environment in the Matsunaga Bay and surrounding areas in Bingo Nada. These marine areas seem to be better in condition within the Seto Inland Sea. Zostera marina is known as an indicator of the marine environment, of which its habitat in this area has been reported. Geochemical examination of tidal flat sediment, suspended solid (SS) and Z. marina was carried out for environmental evaluation in this sea. The water sample was collected at every 1 hour from spring time to ebb time, and water quality was examined in the field. There was no clear correlation of the values of pH, EC and DO with tide level change. Variation of ORP and SS with the tidal change shows negative correlation. Towards the ebb time, ORP and SS values increased, and at the spring tide these values decreased. Twenty sediment samples were collected at the tidal flat of the Matsunaga Bay and 15 samples in Utsumi island (south of Matsunaga Bay), and 6 samples in Momoshima island were collected. Three clusters can be distinguished on the diagrams of Zn, Pb, Cu versus Fe_2O_3 plots. One cluster distributed on the urban trend and/or the over this trend. The second cluster distributed between the area of urban and normal sediment trends. The other plots scattered in the area below the normal sediment trend. Samples of Zostera marina was collected at the coast of Matsunaga Bay, Yokoshima, Tajima and Momoshima. Z. marina appearance may have been due to the mitigation of the tidal flat of the eastern coast of Momoshima done in late 1980’s to early 1990’s. For multi-element analysis, Z. marina sample was separated into three plant tissues (leaf, stem and root). Z. japonica was also examined with separation into two parts (leaf and root). Concentrations of elements in each tissues of Z. marina showed characteristic distribution. Contents of CaO, MnO and Zn were higher in the leaf relative to those of the stem and the root. P_2O_5 concentrations show higher values in the stem relative to other parts. MnO, Fe_2O_3 and As concentrations were significant in the root relative to other parts. Seasonal change of Zn, CaO and MnO values in the leaf for the Yokoshima samples was apparent. This change can coincide to the growth of Z. marina, suggesting elongation and widening of the leaf and increasing attached creatures on the leaf. Contents of P_2O_5 of Z. marina sample did not show apparent seasonal change indicating steady storage of this element in the stem. The results of these geochemical examination are useful for further understanding of the material circulation in marine environments and can be basic data for monitoring.