1. A document image processing apparatus including a processor, comprising: a character image feature dictionary; a character string clipping portion to clip character images in units of character string composed of a plurality of characters from an inputted document image; an image feature extracting portion to divide the character images in the character string clipped by the character string clipping portion, character by character, and to extract an image feature of the divided respective character images, as the resultant vector; a feature similarity measurement portion to select 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 images in units of character based on the image features of each of the character images extracted by the image feature extracting portion, to prepare a first index matrix of M×N cells where M (M>1, integer) represents a number of characters in the clipped character string, and to prepare a second index matrix having a meaningful character string at a first column of the first index matrix, the meaningful character string being formed by adjusting a candidate character string by application of a lexical analysis using a predetermined language model to the candidate character string composed of a plurality of candidate characters constituting the first column of the first index matrix; an index information storing portion to store the second index matrix prepared by the feature similarity measurement portion, so as to correspond to the inputted document image; and a searching section to search, 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 the feature similarity measurement portion adjusts, based on the lexical analysis, the candidate character string of the first column in the first index matrix into a meaningful candidate character string by replacing a candidate character in the first column by another candidate character in the same row in which the candidate character exists in the first index matrix.