In order to reduce sediment disasters, it is important to assess to what extent the risk increases by watching out current rainfall conditions in an area and to maintain high levels of vigilance and preparation for evacuation. For this purpose, we have developed a visualization system for hazard prediction of sediment disasters. In this system, a value called rainfall index R', having a good correlation with the risk of sediment disasters, is calculated using two kinds of values of effective rain for 1.5h and 72hs, which are carried in Home Page of “Site of prevention of disaster" of Hiroshima Prefecture. The calculated values are automatically plotted on a map of Higashi-Hiroshima city and its surroundings every one hour. Thus, the isopleth map obtained describes intelligibly when and where sediment disasters are likely to occur.