3. A text mining apparatus comprising: a storage unit that stores a set of documents as a text mining object; an analyzing unit that reads and analyzes the document from the storage unit and obtains a sentence structure representing a dependency among words; a similar-structure generating unit that performs predetermined modification operation, including at least change in connection of branches in a graph structure, of the partial structure of the sentence structure obtained by the analysis of the analyzing unit, and generates a similar structure of patterns having a similar meaning; and a pattern detecting unit that uses the similar structure generated by the similar-structure generating unit as an equivalent class of the partial structure on the generation source, and detects the pattern, wherein the similar-structure generating unit 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 in the sentence structure and the plurality of new partial structures to produce new similar structures , and wherein the similar-structure generating unit generates the new similar structures of the sentence structure and sets the new similar structures as an equivalent class of the plurality of new partial structures of the sentence structure.
3. A text mining apparatus comprising: a storage unit that stores a set of documents as a text mining object; an analyzing unit that reads and analyzes the document from the storage unit and obtains a sentence structure representing a dependency among words; a similar-structure generating unit that performs predetermined modification operation, including at least change in connection of branches in a graph structure, of the partial structure of the sentence structure obtained by the analysis of the analyzing unit, and generates a similar structure of patterns having a similar meaning; and a pattern detecting unit that uses the similar structure generated by the similar-structure generating unit as an equivalent class of the partial structure on the generation source, and detects the pattern, wherein the similar-structure generating unit 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 in the sentence structure and the plurality of new partial structures to produce new similar structures , and wherein the similar-structure generating unit generates the new similar structures of the sentence structure and sets the new similar structures as an equivalent class of the plurality of new partial structures of the sentence structure. 5. A text mining apparatus according to claim 3 , further comprising: means for adjusting the operation so that a user determines how similar patterns are identical and detecting the pattern.