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 each block of a document data set in association with confidentiality information that indicates whether or not the block is confidential, the document data set being dividable into a plurality of blocks, each of the blocks being a unit including properties evaluated as having a certain characteristic; acquiring a document data set whose degree of confidentiality is to be determined; determining whether a document data set, including a block similar to each of blocks of the acquired document data set, is stored; in response to a determination that the document data set including the similar block is stored, determining whether the confidentiality information indicating that the block is confidential is assigned to the block in the acquired document data set corresponding to the similar block; in response to a determination that the confidentiality information indicating that the block is confidential has been assigned, determining that the acquired document data set is confidential; determining whether the confidentiality information is assigned to all the blocks; extracting document data subsets that are subsets of the acquired document data set if it is determined there is a block not assigned the confidentiality information; determining whether all the document data subsets of the acquired document data set are non-confidential; and assigning the confidentiality information assigned to the similar block if all the document data subsets of the acquired document data set are determined to be non-confidential.