A sediment trap experiment was conducted for 7 years with sampling at 7 days interval at the Tokyo Bay mouth. The δ15N of the trapped particles showed an increasing trend (0.17 ? y-1) from 1995 to 2001. The trend is larger in summer than in winter particles. During summer, the δ15N shows large variation where maxima increase while the minima stay nearly constant. The summertime elevation in δ15N is due to increased contribution of freshly derived 15N rich particles produced in the inner-bay surface water. The high δ15N in trapped particles lagged one or two weeks behind the >50 mm d-1 rainfall events. Freshwater supplied by heavy rainfall appears to accelerate estuarine circulation and results in increased contribution of inner-bay surface water particles. The increased frequency of the short-term heavy rainfall events in metropolis Tokyo during our observation could be a cause for the 7 year increasing trend of δ15N in the sinking particles.