Proper warning and evacuation are considered to be essential for mitigating sediment-related disaster during a heavy rainfall. For this purpose, scientific and rational rainfall index should be established in accord with the regional geology and topography. A new Rainfall Index, named R', is proposed through the revision of Rainfall Index R_f. Effect of preceding rainfall prior to a sediment-related disaster is taken into account for this newly proposed Rainfall Index R' by utilizing the concept of the effective rainfall. Rainfall records during the serious disaster happened to take place on 29 June, 1999 and 15 September, 1999 in Hiroshima district were taken as the cases to verify the effectiveness of this newly proposed Rainfall Index. As a result of the comparison between R_f and R', it was shown that R' is superior to judge the danger of sediment disaster for identifying an area and time from on-going rainfall.