Most of organic contaminants are highly concentrated at the sediment-water interface in the shallow coastal marine environments. Organic detritus deposited onto the sediment surface is decomposed and mineralized to inorganic nutrients (primarily nitrogen and phosphorus) through metabolic activities of benthic communities, and those regenerated nutrients recycle into the overlying water (Fig. 1). This efflux of nutrients from the sediment has been recognized as an important factor in accelerating eutrophication in coastal marine waters. These processes, however, can hardly be understood yet, and there is no standard procedure for obtaining information on these processes. Therefore, we are working on the measurement techniques for obtaining data sets concerning decomposition and mineralization processes of organic matter (carbon, nitrogen and oxygen) at the sediment-water interface in the coastal area characterized by strong eutrophication, and trying to develop a mathematical model to simulate material flux at the sediment-water interface with obtained data sets.