We organized the final level and goal of rainfall forecast accuracy required for dam pre-release by targeting at multi-purpose dams in Japan and rainfall for the past 16 years, as the use of numerical rainfall forecast for it is becoming more active at dams nationwide. Specifically, we assessed the impact on the index directly related to the judgement of pre-release calculated by inputting observed rainfall as it is or with processing to runoff-dam model. As a result, just several events requiring pre-release happened nationwide per year. They were caused mainly by typhoon body and stationary front in descending order. Required lead time is within a day and a half in Typhoon cases, and ranges to 2-3 days in front cases. This indicates the whole lead time of current Meso-Scale Model should be high accurate and be also extended further. About spatial distribution, it is indicated that current model’s resolution is generally sufficient, and the level of position accuracy should be finer than the one of current forecasting by one digit.