6. 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 generation adjustment unit that generates a first determination item for determining, from a user input, whether or not the structures are identical ones for every type of differences between the sentence structures; a similar-structure determination adjustment unit that generates a second determination item for determining, from a user input, whether or not the structures are identical ones for every type of differences between attribute values; a similar-structure generating unit that performs predetermined conversion operation of a partial structure of the sentence structure obtained by the analyzing unit in accordance with the first determination item generated by the similar-structure generation adjustment unit and generates similar structures having a similar meaning of the partial structure; and a similar-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 frequent pattern by ignoring the difference between the attribute values in accordance with the second determination item of the similar-structure determination adjustment unit, wherein the similar-structure generating unit comprises: means for performing parallel modification of the sentence structure when the first determination item determines the parallel modification, 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 when the first determination item determines the non-directional branching of the directional branch 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 when the first determination item includes replacement of the synonym 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 when the first determination item determines the non-ordering of the ordering trees 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 the equivalent class of the plurality of new partial structures of the sentence structure.
6. 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 generation adjustment unit that generates a first determination item for determining, from a user input, whether or not the structures are identical ones for every type of differences between the sentence structures; a similar-structure determination adjustment unit that generates a second determination item for determining, from a user input, whether or not the structures are identical ones for every type of differences between attribute values; a similar-structure generating unit that performs predetermined conversion operation of a partial structure of the sentence structure obtained by the analyzing unit in accordance with the first determination item generated by the similar-structure generation adjustment unit and generates similar structures having a similar meaning of the partial structure; and a similar-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 frequent pattern by ignoring the difference between the attribute values in accordance with the second determination item of the similar-structure determination adjustment unit, wherein the similar-structure generating unit comprises: means for performing parallel modification of the sentence structure when the first determination item determines the parallel modification, 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 when the first determination item determines the non-directional branching of the directional branch 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 when the first determination item includes replacement of the synonym 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 when the first determination item determines the non-ordering of the ordering trees 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 the equivalent class of the plurality of new partial structures of the sentence structure. 8. A text mining apparatus according to claim 6 , wherein the similar-pattern detecting unit detects a frequent similar pattern.