1. A method executable in a computer for supporting a determination of a degree of confidentiality of a document data set, the method comprising the steps of: storing a document data set assigned (i) confidentiality information indicating whether or not the document data set is confidential, and (ii) characteristic information representing a characteristic of the document data set; acquiring a document data set whose degree of confidentiality is to be determined; determining whether document data sets having characteristic information similar to the characteristic information of the acquired document data set are stored; if it is determined that document data sets having the similar characteristic information are stored, calculating a characteristic information difference between each pair of a confidential document data set and a non-confidential document data set from among the document data sets that have the similar characteristic information and are determined to be stored; judging whether the calculated characteristic information difference is included in the acquired document data set; determining that the acquired document data set is confidential when the calculated characteristic information difference is determined to be included in the acquired document data set; determining that the acquired document data set is non-confidential when the calculated characteristic information difference is determined not to be included in the acquired document data set; obtaining a union of the characteristic information of the plurality of document data sets that have the similar characteristic information and are determined to be stored if it has been determined that a plurality of document data sets having similar characteristic information are stored; and calculating a characteristic information difference between a union of the characteristic information of one or more different confidential document data sets and a union of the characteristic information of one or more non-confidential document data sets in response to obtaining the two unions; wherein at least one the unions includes at least two different document datasets.