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 classifies a plurality of character string element regions constituting said document image into small regions and large regions based on a difference of a feature quantity among said character string element regions, sets a feature section for each candidate region that represents a candidate of said index region among said small regions, as for a style type different in feature from a corresponding related large region that represents the large region related to the intended candidate region among a plurality of style types, said feature section including a feature of said intended candidate region but not including a feature of said corresponding related large region, and extracts the candidate region determined as an index based on a group size of at least one or both of the large regions and the candidate regions having the feature included in said set feature section, as said index region.