1. A reading order determination method for determining a reading order of characters by a computer, wherein the computer is accessible to a character information storage unit for storing a list of character information, the list containing character information made up of text data and character region data of each character extracted from image data by character recognition processing aligned and listed in a reading order of characters, and a line information storage unit for storing a list of line information, the list listing line information made up of a line box that surrounds a set of characters continuously aligned in the same direction and an alignment direction of characters in the line box in the alignment order of lines, the method comprising: (a) detecting line information having a line box containing a region indicated by character region data of added character information from the list of line information in response to a request for addition of character information to the list of character information; (b) obtaining a subset of character region data from the list of character information, wherein the character region data indicating a region contained in the line box of the line information detected by said detection means; (c) rearranging each character region data in the subset according to a position with respect to the alignment direction of characters of the line information that has been detected; and (d) determining a reading order of characters in the list of character information by updating the list of character information based on an alignment order of character region data in the subset.