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: 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; 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.