Erosion process of soft cohesive material which deposited naturally on a river bed is the objective of the present paper. Extensive series of experiments, the condition of which were set systematically, were conducted to evaluate the erosion rate by running water in a closed conduit. The test sample was the mixture of silica sand. clay and water. the composition of which was set to be a variable. The main factors which govern this process were found to be the clay content ratio, the water content ratio, a frictional velocity of flow, a water temperature, and the size of silica sand. The effects of these parameters on the erosion rate were evaluated quantitatively. The correlation between the erosion rate and the adhesive strength of the sediment was also investigated by conducting the soil mechanical test. Erosion rate formula was also derived on the basis of experimental results.