Abstract: Storm water pumps drain storm water in a pump well. In order to control the storm water pumps, a radar rain gage and ground raingages are set. Measurement data from the radar rain gage and the ground rain gages are supplied to a data processing unit. The data processing unit calibrates rainfall distribution data representing a two-dimensional rainfall distribtuion state obtained by the radar rain gate by the measurement data from the ground rain gages, and forecasts a rainfall in a predetermined time from the present from several sets of the calibrated rainfall distribution data. The data processing unit performs runoff analysis corresponding to characteristics of a drainage basin on the basis of the forecast rainfall to calculate a rainfall flow, thereby forecasting a flow of storm water flowing in the pump well.