1. A method of extracting an important item from an input document including at least one document segment, comprising the steps of: (a) generating, for the respective at least one document segment, document segment vectors having values relating to occurrence frequencies of terms occurring in the respective at least one document segment as component values; (b) generating a square sum matrix from the document segment vectors; (c) calculating eigenvectors and eigenvalues of the square sum matrix; and (d) selecting the important item of the input document from the calculated eigenvectors and eigenvalues of the square sum matrix.