1. A text mining apparatus comprising: means for generating a sentence structure from an input document, the sentence structure representing a dependency among words; means for generating a similar structure of patterns having a similar meaning of a partial structure of the sentence structure by performing predetermined conversion operation, including at least change in connection of branches in a graph structure, of the partial structure; and means for determining the patterns having the similar meaning as the identical pattern and detecting the patterns, wherein the means for generating the similar structure comprises: means for performing parallel modification of the sentence structure, the parallel modification being structure modification including new branch generation for a particular one of nodes corresponding to the words put in a parallel relationship in the sentence structure so that the particular one is connected to each node connected by a branch from the node put in the parallel relationship for the particular one, said means for performing parallel modification of the sentence structure generating the similar structure; means for generating a plurality of new partial structures of the sentence structure from the partial structure and the similar structure; means for performing non-directional branching of a directional branch of the sentence structure and the plurality of new partial structures to produce new similar structures; means for replacing a synonym in the sentence structure and the plurality of new partial structures by referring to a synonym dictionary to produce new similar structures; and means for performing non-ordering of ordering trees of the sentence structure and the plurality of new partial structures to produce new similar structures, and wherein the means for generating the similar structure uses the new similar structures as an equivalent class of the plurality of new partial structures of the sentence structure.