1. A classifying method of classifying an article into one of a plurality of groups based on an image of the article, comprising: determining an evaluation method for obtaining an evaluation value of an image by using at least some of sample images, the sample images indicating images of a plurality of samples each of which has already been classified into one of the plurality of groups; obtaining evaluation values for the sample images by the determined evaluation method; changing the evaluation method so as to increase a degree of dissimilarity in an evaluation value range for sample images between the plurality of groups by changing a evaluation value of at least one sample image having a singular evaluation value among the sample images; and obtaining an evaluation value for the image of the article using the changed evaluation method, and classifying the article into one of the plurality of groups based on the evaluation value for the image of the article.