1. A document image processing apparatus comprising: a memory for storing a document image; and a controller for controlling extraction of an index region from said document image, wherein said controller is configured to i) classify a plurality of character string element regions constituting said document image into small regions and large regions, ii) determine each small region positioned just before said large region according to a reading order as a first candidate, as a first determining process, iii) determine at least one part of said first candidates as a first index, by performing an evaluating process to evaluate whether or not each said first candidate is an index, based on a difference in feature from the related large region, with respect to each said first candidate, as a first evaluating process, iv) determine each small region positioned just before said first index according to the reading order as a second candidate, as a second determining process, v) determine at least one part of said second candidates as a second index, by performing an evaluating process to evaluate whether or not said second candidate is the index, based on a difference in feature from the related first index, with respect to each said second candidate, as a second evaluating process, and vi) extract the small regions determined as said first index and said second index, as said index region.