PURPOSE: To improve operability by providing versatility, improving the preservation of data and the precision of a prediction result and grasping the reliability of the prediction result.

CONSTITUTION: A prediction expression, a prediction period, prediction time, the number of data, a data level value, an alarm value for the prediction result and a correction value, which are inputted by an input means 13 to a set value file 11, are referred to while past prediction data is read and the prediction result is calculated S1-S3. Then, reliability on the obtained prediction result is evaluated S4.