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, wherein a position-based correlation value is set for each of the elements in the second index matrix, and the searching section comprises: an index matrix search processing portion for searching the index information storing portion for the second index matrix in units of search character constituting the search keyword to detect the second index matrix containing the search characters, and storing in a storing portion, information of matching position of search characters in the second index matrix together with information of the document images having the second index matrix; a degree-of-correlation calculating portion for calculating a degree of correlation between the search keyword and the second index matrix by accumulating correlation values of the respective search characters according to the information of matching position stored in the storing portion; and an order determining portion for determining a take-out order of document image based on the calculated result of the degree-of-correlation calculating portion.