To provide a device for predicting the amount of flowout of rainfall which can predict the amount of future rainfalls by using such a predicting method that can adapt itself to various conditions of rainfalls and, as a result, can realize a reliable operation supporting system for rainwater sewage pump station.

The device for predicting flowout of rainfall is provided with a raining state index calculating means 2 which calculates raining state indexes for classifying raining states from measurement data obtained from rain gauges 1 distributed in a basin at prescribed time intervals and a rainfall predicting method selecting means 3 which selects an adaptive predicting method by using the value of a calculated raining state index by correlating in advance adaptive rainfall predicting methods for every raining state classified by the values of the raining state indexes in accordance with each raining state. The device is also provided with a rainfall predicting means 4 which predicts the amount of rainfall form measurement data obtained from the gauges 1 at every prescribed time interval in accordance with a selected predicting method and a means 5 for predicting the amount of flowout of rainfall which predicts the flowout of rainfall form the basin from the value of the predicted rainfall by using a prescribed technique.