1. A document image processing apparatus for use with a document image comprising a plurality of character string element regions, the document image processing apparatus comprising: a memory for storing the document image; and a controller for controlling extraction of an index region from said document image, wherein said controller is configured to: for each character string element region, identify the character string element region as a large region or a small region based on at least one of a size property and a character property of the character string element region; for each small region, identify the small region as an index candidate when a region immediately following the small region is a large region, and identify the large region immediately following the index candidate region as a related text region; for each index candidate region, compare formatting of the index candidate region to formatting of the related text region and identify a formatting property that is different between the index candidate region and the related text region as a different formatting property; for each index candidate region, set an evaluation criterion such that a value of the index candidate region for the different formatting property satisfies the evaluation criterion and a value of the related text region for the different formatting property does not satisfy the evaluation criterion; for each index candidate region: set the index candidate region as the focused index candidate region and set the different formatting property of the focused index candidate region as the focused different formatting property; calculate at least one of a number of similar index candidate regions and a number of similar large regions, wherein: the number of similar index candidate regions is a total number of index candidate regions among the index candidate regions, except for the focused index candidate region, that satisfy the evaluation criterion for the focused different formatting property; and the number of similar large regions is a total number of large regions that satisfy the evaluation criterion for the focused different formatting property; calculate an index evaluation degree based on at least one of the number of similar index candidate regions and the number of similar large regions; identify index regions from among the index candidate regions based on the index evaluation degree of each index candidate region.