PURPOSE: To judge the soundness of equipment and the soundness of measured signal accurately.

CONSTITUTION: The measured signals from a plurality of sensors, which have measured the same condition of a plant, and the measured signal which are obtained by measuring the state of a plurality of the same kind of equipment, are inputted into an A/D converter device 2 as the plant signals (a). The A/D converter device 2 samples the plant signals at a constant period and obtains the statistic characteristic quantities between the plant signals as the set of the data in the multi-dimensional state space. A measured-signal classifying device 4 detects the similarity of the plant signals based on the distance between the data sets and classifies the plant signals in response to the similarity. An abnormal-signal detecting and reporting device 6 selects the abnormal signal among the plant signals and reports the signal.