For the reduction of debris-flow disasters spawned by heavy rainfall, the influence of geomorphic quantities of mountainous small drainage basin and some rainfall indices on geomorphic threshold for the debris-flow arrival and arrival ratio of debris flow to the basin outlet were investigated for three disasters with different rainfall patterns (occurred in 1999, 2014, and 2018) on the granite slopes in and around Hiroshima City. The geomorphic thresholds for the debris-flow arrival and for the arrival ratio of debris flow expressed as function of basin area and basin relief ratio could be shown as a function of any of the three types of rainfall indices : the 6-hour rainfall amount, the half-life 8-hour effective rainfall, and R'-value. In this analysis, a new calculation method was used to minimize the overestimation of the effective rainfall.