This study introduces a new index, the unprecedented rainfall index, TP, for evaluating sediment disaster vulnerability. When we trace back to the past rainfall events, we find a time at which the rainfall magnitude is equal to or greater than the magnitude of the current rainfall. Such time is treated as a candidate for TP. Because the rainfall magnitude can be evaluated from various aspects, various TP candidates are derived. Among them, the oldest TP candidate is defined as TP. Consequently, in comparison with the rainfall magnitudes during the period from TP (exclusive) to the start time of the current rainfall event, the current rainfall has the largest magnitude at least in one of the various aspects. The new index was applied to previous sediment disasters, and results indicated that TP has a potential to be effectively used for a warning system against sediment disasters because it can be directly compared to occurrence times of previous disasters which are famous in each community. Based on the results, this study suggested a novel critical value, TC, for sediment disaster vulnerability, which is defined as the occurrence time of reduction in resistance against slope failures and debris flows.