1. A text mining apparatus for performing text mining using, as targets, a plurality of text data pieces including a text data piece generated by computer processing, confidence being set for each of the text data pieces, the text mining apparatus comprising: a processor, an inherent portion extraction unit that, for each of the text data pieces, extracts an inherent portion of the text data piece relative to another of the text data pieces using the processor; an inherent confidence setting unit that, for each inherent portion of each of the text data pieces relative to another of the text data pieces, sets inherent confidence that indicates confidence of the inherent portion, using the confidence that has been set for each of the text data pieces; and a mining processing unit that performs 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 a numerical value less than or equal to one is set as the confidence for each of the text data pieces, and 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 setting unit sets the inherent confidence 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.
4. The text mining apparatus according to claim 1 , wherein a text data piece generated by speech recognition is used as the text data piece generated by computer processing, the text mining apparatus further comprising: a confidence setting unit that sets confidence for the text data piece generated by speech recognition, using a word graph or an N-best word sequence obtained when performing speech recognition.