1. A document retrieval system, comprising: a document database for storing data for a plurality of documents; an arithmetic unit that: includes a numeric value data reading unit configured to read, from the data on the documents stored in the document database, numeric value data for which numeric value intervals are to be generated; calculates indices used for indexing numeric values and texts in each of the documents stored in the document database, 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 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 document; receives a designation of a document as a retrieval input; and computes a similarity between the designated document and each of the documents stored in the document database by use of the indices; and a numeric value distribution percentage designating unit configured to designate the numeric value intervals from distribution percentages of numeric values based on a distribution of the numeric value data, wherein the arithmetic unit includes a numeric value range designating unit configured to generate the numeric value intervals based on distribution percentages inputted by the numeric value distribution percentage designating unit; and the numeric value distribution percentage designating means 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.