This reportis experimental research on soil erosion occuring at the surface of sloped farmland. The subject of this research are discussed the phenomenon of soil erosion to change the cover ratio at the surface of sloped farmland accompanying growth of the crops. The crops using this research are corn, soybean and grass. The main results are as follows: 1) The run-off ratio of rainfall on the sloped farmland shows a decrease accompanying growth of crops, and the volume of percolated water into the ground shows a increase. This tendency notably appears in the cover ratio 40-50%. 2) The relation between the decrease of soil dencity and the increase of cover ratio are expresed by exponential function. 3) The effects for prevention of soil erosion by growth of crops are remakable in the case of the cover ratio 40-50%, and in the case of the grass than soybean and corn. But, its effects have a limit according to the amounts of rainfall in the case of soybean and corn.