For the purpose of making clear the mechanism of soil erosion, rate of eroded soil in keeping with the change of the rainfall intensity, slope length and slope steepness was measured about four kind of soils. Erosion rate almost exponentially increases relatively to slope length, and also the relationship between erosion rate and slope steepness are almost linear so far as rill doesn't happen. At so rill happens, the concentration of soil in water is almost constant without reference to the rainfall intensity and slope length, but increases with slope steepenss increases. Result of multiple regression analysis, we obtained following relation : A=0.97K(L/5)^<1.40>(S/0.174)^<1.21>(278+100log_<10>I)I^2