To elucidate production process of N2O in marine sediments, firstly, a continuous-flow sediment-water system with 15N tracer (15N-NO3- addition) under simulated in situ condition was applied in an estuarine sediment (Tama Estuary). Both in February and May, net N2O efflux from the sediment was observed. Production of 15N enriched N2O (molecular weight of 45 and 46, expressed as 45N2O and 46N2O, respectively, hereafter) in the sediment indicates N2O formation by denitrification using 15N-NO3- diffused from the overlying water. Efflux of 44N2O from the sediment also occurred in February, while overlying 44N2O was consumed in the May sediment. In the model proposed by the studies in soils, contribution of nitrification and denitrification to N2O production have been evaluated by assuming 15N contents in the produced N2O to be equal to its source compounds, i.e. NH4+ and NO3- (N2). However, N2O produced in the February sediment was highly enriched in 15N compared with interstitial NH4+ and N2 evolved from the sediment, due to higher fraction of 45N2O plus 46N2O in the N2O. Also, 44N2O consumption in the May sediment was not accounted for the previous model. These results can not be explained without considering simultaneous N2O production/consumption during denitrification as an intermediate product. Based on a new model including the above process, contribution of denitrification to net N2O production in the February sediment was more than 69-76%, assuming 44N2O reduction by denitrification to be insignificant. Relative contribution of both processes in May could not be evaluated under my experimental condition. Indirect contribution of nitrification, however, should be important in both seasons, because denitrification significantly depended on NO3- supply from nitrification (52% in February and 37% in May). Secondly, detailed profiles of N2O, O2 and nitrogenous nutrients were observed in sediments from estuary to open ocean using the whole core squeezer. These profiles are categorized into four types. This classification suggests that change in benthic N2O metabolism in the order of N2O production predominantly by nitrification, N2O production by nitrification and/or denitrification, and N2O consumption by denitrification is caused by increase in organic matter supply and temperature and decrease in bioirrigation and overlying water O2 and NO3-. Finally, variation of benthic N2O flux from estuary to open ocean was evaluated using the results in this study and reported values. The magnitude of benthic N2O flux decrease from nearshore to open ocean, i.e., -400 to 11240 nmolN m-2 h-1 in estuarine and coastal areas, -86 to 92 nmolN m-2 h-1 in marginal sea and <0.2 to 0.23 nmolN m-2 h-1 in open ocean. The decrease in organic deposition and biological activity in the sediment from nearshore to open ocean are likely important for large-scale variation of microbial N2O metabolism in the sediment.