1. A document image processing apparatus comprising: a character image feature dictionary for storing image features of character images in units of character; a character string clipping portion for clipping character images in units of character string composed of a plurality of characters from an inputted document image; an image feature extracting portion for extracting an image feature of each character image which is obtained by dividing character images of character string clipped by the character string clipping portion, for each of the characters; a feature similarity measurement portion for selecting N (N>1, integer) pieces of character images in descending order of degree of similarity of image feature as candidate characters, from the character image feature dictionary which stores image features of character image in units of character based on the image features of each of the character images extracted by the image feature extracting portion, preparing a first index matrix of M×N cells where M (M>1, integer) represents a number of characters in the clipped character string, and preparing a second index matrix of character strings including a meaningful character string which is formed by adjusting candidate character strings by application of a lexical analysis using a predetermined language model to the candidate character strings composed of a plurality of candidate characters constituting a first column of the first index matrix; an index information storing portion for storing the second index matrix prepared by the feature similarity measurement portion, so as to correspond to the inputted document image; and a searching section for searching, in a searching operation, the index information storing portion in units of search character constituting a search keyword of an inputted search formula, to take out the document image which includes the second index matrix containing the search character.