In recent years, refinement in the accuracy of rainfall forecasting has been called for in Japan because of extremely heavy rainfalls that have occurred in various areas. For dam operation in particular, accurate rainfall forecasting would be greatly beneficial in allowing for the release of water prior to a forecast heavy rainfall event, which would make it possible to increase water control and water use capacities. Comparison of the cumulative rainfall forecasts calculated by using the numerical weather forecast models, GSM and MSM, with actually measured rainfall revealed that the accuracy of the two models was good for long period rainfall events such as those caused by typhoons. The accuracy of the MSM rainfall forecast for localized heavy rainfall that occurs in a short period was also found to be good. A calibration method, which can be used in disaster mitigation and dam operations, was suggested thorough clarification of the error ranges of rainfall forecasting.