1. A method of retrieving documents having a common topic and of classifying the documents into a first document set having a first set of feature values and a second document set having a second set of feature values, the method comprising: retrieving a related third document set on the basis of a predetermined term list; constructing a third set of feature values by calculating feature values for each document in the third documents set; and classifying documents in the third document set into the first document set and the second document set according to: (a) a discriminant using the first set of feature values and the third set of feature values, and (b) a discriminant using the second set of feature values and the third set of feature values.