PURPOSE:To make the reproducibility of data excellent and to reduce data amount by extracting and recording only the data to be considered to be necessary for the evaluation. CONSTITUTION:A data preservation device 1 is provided with a temporary preservation device 3 preserving plant data 2 temporarily, a data division device 4 dividing this preserved data into the prescribed amount, a data approximate device 5 determining the approximate expression by defining the displacement of data within this divided data range as a time function, a deviation calculation device 6 determining the deviation between this approximate expression and actual plant data, a preservation decision processing device 7 comparing this deviation with the preliminarily set threshold, requesting for the preservation when it is other than the threshold range and updating the data division by this decision and a data preservation device 8 preserving the data in accordance with this preservation request, and records a time series signal obtained from a plant.
