13. The text mining device according to claim 2 , wherein the differential processing unit further comprises a differential block extracting unit that extracts, from the document data, a block, which includes the differential element extracted by the differential element extracting unit and comprises an element larger than the differential element, as a differential block, and the differential processing unit extracts a plurality of the differential block from the document data.
13. The text mining device according to claim 2 , wherein the differential processing unit further comprises a differential block extracting unit that extracts, from the document data, a block, which includes the differential element extracted by the differential element extracting unit and comprises an element larger than the differential element, as a differential block, and the differential processing unit extracts a plurality of the differential block from the document data. 14. The text mining device according to claim 13 , wherein the differential block extracting unit extracts the plurality of the differential block on a sentence structure basis, a partial structure of the sentence structure basis or a sentence basis.