1. A method of classifying a given input document into at least one document class, the method comprising the steps of: (a) selecting terms for use in the classification, from among terms that occur in the input document; (b) dividing the input document into document segments in predetermined units; (c) generating document segment vectors whose components are values relevant to occurrence frequencies of the selected terms occurring in the document segments, and generating a document vector in which all of the document segment vectors are added together; (d) calculating a similarity between the input document and each class using information saved for every document class; (e) correcting the similarity to each class; and (f) determining the class to which the input document belongs in accordance with the corrected similarity to each class.