9. A non-transitory computer-readable recording medium recording a program for causing a computer device to perform text mining using, as targets, a plurality of text data pieces including a text data piece generated by computer processing, the program including instructions that cause the computer device to perform the steps of: (a) setting confidence for each of the text data pieces; (b) for each of the text data pieces, extracting an inherent portion of the text data piece relative to another of the text data pieces; (c) for each inherent portion of each of the text data pieces relative to another of the text data pieces, setting inherent confidence that indicates confidence of the inherent portion, using the confidence that has been set for each of the text data pieces; and (d) performing text mining on each inherent portion of each of the text data pieces relative to another of the text data pieces, using the inherent confidence, wherein in the step (a), a numerical value less than or equal to one is set as the confidence for each of the text data pieces, and in the step (c), in a case of setting the inherent confidence for each inherent portion of one of the text data pieces relative to another of the text data pieces, the inherent confidence is set by multiplying the confidence that has been set for the one text data piece by a value obtained by subtracting the confidence that has been set for the other text data piece from one.
9. A non-transitory computer-readable recording medium recording a program for causing a computer device to perform text mining using, as targets, a plurality of text data pieces including a text data piece generated by computer processing, the program including instructions that cause the computer device to perform the steps of: (a) setting confidence for each of the text data pieces; (b) for each of the text data pieces, extracting an inherent portion of the text data piece relative to another of the text data pieces; (c) for each inherent portion of each of the text data pieces relative to another of the text data pieces, setting inherent confidence that indicates confidence of the inherent portion, using the confidence that has been set for each of the text data pieces; and (d) performing text mining on each inherent portion of each of the text data pieces relative to another of the text data pieces, using the inherent confidence, wherein in the step (a), a numerical value less than or equal to one is set as the confidence for each of the text data pieces, and in the step (c), in a case of setting the inherent confidence for each inherent portion of one of the text data pieces relative to another of the text data pieces, the inherent confidence is set by multiplying the confidence that has been set for the one text data piece by a value obtained by subtracting the confidence that has been set for the other text data piece from one. 11. The non-transitory computer-readable recording medium according to claim 9 , wherein in the step (b), for each word constituting each of the text data pieces, the degree to which the word corresponds to an inherent portion of the text data piece relative to another of the text data pieces is calculated using the confidence that has been set for each of the text data pieces, and an inherent portion of the text data piece relative to the other text data piece is extracted based on the calculated degree.