A sensor data input device obtains a plurality of sensor data sequences, each indicating time-series sensor values generated by measuring a work of a working subject using a sensor. A class data generator determines a plurality of intervals obtained by temporally dividing each sensor data sequence based on the sensor values, determines classes of the intervals, each class indicating a type of temporal variations in the sensor values included in one of the intervals, and generates a plurality of first class data sequences for each sensor data sequence, each first class data sequence indicating the intervals and the classes of the sensor data sequence. A class data linker associates the corresponding intervals having an identical class with each other, among the plurality of first class data sequences. A determiner calculates characteristic values of the intervals associated with each other.