14. A text mining method comprising: a step of analyzing a document from a storage unit that stores a set of documents as a text mining object and obtaining a sentence structure representing a dependency among words; a step of generating, from a user input, a first determination item for determining whether or not the structures are identical ones for every type of differences between sentence structures; a step of generating, from a user input, a second determination item for determining whether or not the structures are identical ones for every type of differences between attribute values; a step of performing predetermined modification operation of the partial structure of the sentence structure obtained by the analyzing unit and generating, using a computer, a similar structure having a similar meaning of the partial structure in accordance with the generated first determination item; and a step of using the generated similar structure as an equivalent class of the partial structure on the generation source and detecting the pattern by ignoring the difference between the attribute values in accordance with the second determination item, wherein the step of generating the similar structure comprises: a step of 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 step of performing parallel modification of the sentence structure generating the similar structure; a step of generating a plurality of new partial structures of the sentence structure from the partial structure and the similar structure; a step of 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; a step of replacing a synonym of the sentence structure and the plurality of new partial structures by referring to a synonym dictionary when the first determination item determines the synonym replacement to produce new similar structures; and a step of performing non-directional branching of ordering trees of the sentence structure and the plurality of new partial structures when the first determination item determines the non-directional branching of the ordering trees to produce new similar structures, and thereby the step of generating the similar structure generating the new similar structures of the sentence structure and setting the new similar structures as an equivalent class of the plurality of new partial structures.
14. A text mining method comprising: a step of analyzing a document from a storage unit that stores a set of documents as a text mining object and obtaining a sentence structure representing a dependency among words; a step of generating, from a user input, a first determination item for determining whether or not the structures are identical ones for every type of differences between sentence structures; a step of generating, from a user input, a second determination item for determining whether or not the structures are identical ones for every type of differences between attribute values; a step of performing predetermined modification operation of the partial structure of the sentence structure obtained by the analyzing unit and generating, using a computer, a similar structure having a similar meaning of the partial structure in accordance with the generated first determination item; and a step of using the generated similar structure as an equivalent class of the partial structure on the generation source and detecting the pattern by ignoring the difference between the attribute values in accordance with the second determination item, wherein the step of generating the similar structure comprises: a step of 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 step of performing parallel modification of the sentence structure generating the similar structure; a step of generating a plurality of new partial structures of the sentence structure from the partial structure and the similar structure; a step of 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; a step of replacing a synonym of the sentence structure and the plurality of new partial structures by referring to a synonym dictionary when the first determination item determines the synonym replacement to produce new similar structures; and a step of performing non-directional branching of ordering trees of the sentence structure and the plurality of new partial structures when the first determination item determines the non-directional branching of the ordering trees to produce new similar structures, and thereby the step of generating the similar structure generating the new similar structures of the sentence structure and setting the new similar structures as an equivalent class of the plurality of new partial structures. 16. A text mining method according to claim 14 , wherein the frequent similar pattern is detected.