A study was carried out on the sediment quality of the sea off Fukuyama, the Seto Inland Sea, Japan to evaluate the environmental conditions. Sediment temperature, oxidation-reduction potential, pH, water content, loss on ignition, acid-volatile sulfide concentration, and sediment oxygen demand were measured, along with the hydrogen sulfide concentration in the interstitial water of the sediment. Concentrations of dissolved oxygen, hydrogen sulfide and ammonia in overlying water of the sediment were measured. Ammonia concentration of the surface water was also determined. It revealed that the quality of overlying water of the area is not so deteriorated as to impart serious damage to benthic oraganisms. However, sediment of the area, in particular off the Ashida River, was rich in organic content and sulfide, and hence inhabitable by benthic organisms.