To determine the effects of environmental management and the pollution load from land in the Seto Inland Sea, the water quality has been monitored for many years by the Ministry of Environment, Japan. By using this observed data, we proposed an index of water quality control in the coastal area, and clarified the differences in characteristics of various areas in the Seto Inland Sea. Consequently, it was shown that the effect of pollution load from land varied from area to area. This suggests that the degree of water quality improvement by control of total phosphorus and nitrogen loads differs from area to area in the Seto Inland Sea.