Sediment qualities were analysed at 15-20 stations in the Seto Inland Sea during four cruises in October, 1993, January, April and June, 1994. Surface sediment was collected at each station, and temperature, pH, oxidation-reduction potential, water content, acid volatile sulfide and ignition loss of the collected sediment were determined along with dissolved oxygen concentration in the overlying water of the sediment. Seasonal and regional variations of sediment quality were reported.