PURPOSE: To forecast with a high accuracy a variation of a flow rate and a water level of a sewage drain, a river, etc. by calculating a forecasting value based on two or more different forecasting models, and executing an addition by putting a weight to be changed in accordance with a variation of a system.

CONSTITUTION: A measuring mechanism 100 contains a sensor means, measures a water quantity, a water level, or a rain quantity, a flow rate, etc. of a sewage drain, a river, etc., inputs them to a data collecting mechanism 102, and stores them in a storage device 104. It is read out by a time series data read-out mechanism 106, and each forecasting value is calculated and outputted based on a forecasting model generated statistically by a static operation mechanism 10, and also based on a forecasting model generated physically by a physical operation mechanism 110. An operation mechanism 112 puts a weight to each forecasting value in accordance with a variation of input information by an output of a weight changing mechanism 116, and calculates a forecasting value 114 by a weighted average.