To predict inflows and outflows with high precision as to various dams and ranges of the volume of water without depending upon a skilled operator by complementing deficit or abnormal data with normal data nearby data used for prediction or the gradient of the nearby normal data if deficit or abnormal data are generated in the data used for the prediction.

If the inflow data used for the prediction include deficit data or abnormal data, the deficit data or abnormal data are complemented by complementation of 0th or 1st order. Here, the complementation of 0th order is performed by holding the nearby normal data as they are. The complementation of 1st order, on the other hand, complements the deficit or abnormality of the data by generating a linear function according to the gradient of the normal data right nearby the deficit or abnormality if the deficit or abnormality is generated in the data. In this case, the deficit is judged when there are not data which are transmitted periodically. Further, the abnormality is judged from the differential value, etc., of the inflow data when the inflow data have an unreasonable value or are much different in value from the nearby normal data.