11. The text mining device according to claim 3 , wherein the statistical processing unit further comprises a characteristic degree calculating unit that calculates a degree of characteristic of the differential elements in the difference set extracted from predetermined pieces of the first document data as a characteristic degree of the differential element, against respective differential elements included in the plurality of the difference set, the characteristic degree calculating unit calculating the characteristic degree in accordance with the differential score of the differential element.
11. The text mining device according to claim 3 , wherein the statistical processing unit further comprises a characteristic degree calculating unit that calculates a degree of characteristic of the differential elements in the difference set extracted from predetermined pieces of the first document data as a characteristic degree of the differential element, against respective differential elements included in the plurality of the difference set, the characteristic degree calculating unit calculating the characteristic degree in accordance with the differential score of the differential element. 12. The text mining device according to claim 11 , wherein the characteristic degree calculating unit calculates based on a summation of the differential score of the differential elements in the difference set extracted from the plurality of the first document data and a summation of the differential score of the differential elements in the difference set extracted from predetermined pieces of first document data, when calculating the characteristic degree of the differential element against each differential element included in the difference set.