PROBLEM TO BE SOLVED: To determine a modification in a portion, such as only one character or only a numeric character, of a document to be excerpted during excerption.SOLUTION: The present invention comprises: cutting a character string segment out of one sentence of an input document; determining a start point of the character string segment; making a digest, in which a character string corresponding to a character string segment for each predetermined number of characters from the start point has been converted into a hash function, slide by a predetermined number of characters, and storing a document ID and a digest group of the digest in a digest DB; reading out the digest from the digest DB; and determining that a modification in the size of a predetermined window size w or less is performed, when providing the window size w and an allowable error at the start position of a character string segment, in the case that a character string segment is detected that has a digest in common with that of the same document in a position separated by W+.
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