6. A document retrieval method comprising: receiving a designation of a document as a retrieval input; reading from data on documents stored in the document database, numeric value data for which numeric value intervals are to be generated; calculating a similarity between the document designated as the retrieval input and each of the documents stored in the document database by use of indices of the designated document and indices of each document stored in the document database, the indices used for indexing numeric values and texts in a corresponding document, each of the indices used for indexing the text being a group of a term constituting the text and a frequency of the term in the corresponding document, each of the indices used for indexing the numeric value being a group of a label describing a feature represented by the numeric value, an interval including the numeric value, and a frequency of the numeric value in the corresponding document; presenting the documents stored in the document database in order of similarity; deciding the numeric value intervals from distribution percentages of numeric values based on a distribution of the numeric value data; and generating the numeric value intervals based on distribution percentages inputted by the numeric value distribution percentage designating unit; wherein the generating the numeric value intervals generates numeric value intervals whose numeric value widths are so adjusted that each of the numeric value intervals includes an equal number of numeric values.
7. The document retrieval method according to claim 6 , wherein the interval constitutes a set E of intervals in which each element in a set D of numeric values representing a feature A is included in at least any one of the intervals.