1. A character string anonymizing apparatus, comprising: a storage device which stores a plurality of pieces of text data each including a character string; a data linking unit which classifies each of the plurality of pieces of text data into a plurality of kinds of data in accordance with a classification condition; a morpheme analysis unit which extracts a plurality of words included in each piece of linked data classified into a same kind by the classification among the plurality of pieces of text data; an evaluating unit which extracts, among word sets including at least one of the extracted words, a word set in which the number of the pieces of linked data including all words forming each of the word sets is at least equal to a threshold; an anonymizing unit which anonymizes, among words included in a character string included in each of the plurality of pieces of text data, a word matching at least some of the extracted words and not matching words forming the extracted word set; and an output processing unit which outputs text data including the anonymized word to an output device.