1. A text mining method for extracting features of documents using a term-document matrix consisting of vectors corresponding to index terms representing the contents of the documents, wherein contributions of the index terms act on respective elements of the term-document matrix, said method comprising: a basis vector calculating step of calculating a basis vector spanning a feature space, in which mutually associated documents and terms are located in proximity with each other, based on a steepest descent method minimizing a cost; a feature extracting step of calculating a parameter for normalizing the features using the term-document matrix and the basis vector and extracting the features on the basis of the parameter; and a term-document matrix updating step of updating the term-document matrix to a difference between the term-document matrix, to which the basis vector is not applied, and the term-document matrix, to which the basis vector is applied.