PURPOSE: To reduce processing load for discriminating a work by providing an upper/lower limit threshold value changing means to change upper/lower limit threshold values so as to dissolve overlap of both ranges, under the condi tion in which they are discriminated to be overlapped.

CONSTITUTION: Feature values of a plurality of works for teaching are extracted, and the feature value of the highest work discriminating accuracy is selected as the work discriminating feature value from the extracted feature values, under the condition the work discriminating accuracy based on the previously set feature value in the extracted feature values is not high. Upper/ lower limit threshold values are obtained based on the mean value and dispersion of the selected feature value, and it is discriminated by a discriminating part 5 whether the ranges based on the upper/lower limit threshold values obtained from every sort of work are overlapped or not. Under the condition they are judged to be overlapped, the upper/lower limit threshold values are changed by an upper/lower limit threshold value changing means 8a so as to dissolve overlap of both ranges, and the objective work is discriminated based on the selected work discriminating feature value and the finally obtained upper/ lower threshold values.