For the purpose of making clear the mechanism of soil erosion, rate of eroded soil in keeping with the change of slope steepness and rainfall intensity was examined. The relationship between elapsed time and erosion rate were almost linear. This shows erosion rate is uniform during duration of rain. Erosion rate increases as the rainfall intensity and slope steepness increase. The concentration of soil in water also increases with the rainfall intensity and slope steepness increase. From the results of analysis of size distribution of eroded soil, fine soil particle is eroded easily. Results of multiple regression analysis, we obtained following relation E=0.18 I^<1.30>S^<0.66>.