This study aimed to clarify the possible role of floodplains in biogeochemical nitrogen cycles based on dynamics of inorganic nitrogen species in groundwater. Samples collected from groundwater of the floodplain in the middle reach of the Tama River, Nagata district, were analyzed for dissolved oxygen, dissolved organic carbon, major ions, nitrous oxide and nitrogen stable isotope ratio of nitrate. There were some observed wells where the dissolved oxygen concentration in groundwater was kept low through the observation period. In groundwater with lower dissolved oxygen concentration, nitrate decreased to the detection limit mainly in the summer season. Furthermore, nitrous oxide decreased to below the atmospheric equilibrium concentration (〜10nM) in the season of nitrate decline. In aerobic groundwater, nitrate is the predominant inorganic nitrogen species and concentration of nitrous oxide is known to be higher than the atmospheric equilibrium. However, floodplain groundwater was sink for both nitrate and nitrous oxide. Thus this study proves that unconfined groundwater in floodplain can perform an important role as nitrogen sinks for material cycles in river catchments.