At the confluence, a tributary accompanied with large sediment production and supplying make to increase amount of water and sediment flowing into main river. In addition, rising riverbeds and water levels at the confluence may cause trouble such as sediment floods. In promoting Sabo works, to comprehensively understand the sediment dynamics at the confluence joining a tributary with large sediment production and supplying is crucial issue. In this study, flume experiment considering bed load transport from tributary with large sediment production from mountainous basin were performed. Our flume experiment results suggest that; the water flowing down from upstream of the main river prominently drifts into opposite side of tributary and the riverbed around the side joining tributary shows the deposition trends, and the riverbed around opposite side joining tributary shows washed trend. Furthermore, the trends of riverbed is more prominent as the flow rate ratio of tributary hydrograph to the main river is larger, and the time gap of peak flow between main stream and tributary is larger. The sedimentary and eroded areas near the confluence are similar to the areas of flowing water from the main and tributary. Hereafter, in order to establish a model that easily predicts the flow conditions and riverbed fluctuations at the confluence, it is necessary to quantitatively relate the inflow rate and the flow area of main and tributary, and planner distribution of riverbed height.