1. A text proofreading apparatus comprising: a storage unit that stores pre-proofreading sentences and post-proofreading sentences in association with each other; an acquisition unit that acquires the post-proofreading sentences corresponding to the pre-proofreading sentences, according to characteristics of a proofreading target sentence from the storage unit; a selection unit that selects, from the post-proofreading sentences acquired by the acquisition unit, post-proofreading sentences with degrees of similarity between the proofreading target sentence and the post-proofreading sentences equal to or more than a predetermined threshold value; and a proofreading unit that proofreads the proofreading target sentence, using, out of the post-proofreading sentences selected by the selection unit, a post-proofreading sentence with the highest degree of similarity, wherein the proofreading unit replaces a character string of an overlapping part in the pre-proofreading sentence between a common character string between the proofreading target sentence and the pre-proofreading sentence and a different character string between the pre-proofreading sentence and the post-proofreading sentence, by a character string in the post-proofreading sentence corresponding to the overlapping character string, thereby proofreading the proofreading target sentence.