1. A character string reading device for reading character strings from input image data, comiprising: cut-out recognition means for cutting out a segment corresponding to one character from the image data to perform individual character recognition on each segment; a recognition result buffer for storing a recognition result of said cut-out recognition means; word searching means for searching a word string candidate corresponding to a combination of character candidates in said recognition result buffer; a word string candidate buffer for storing a search result of said word searching means; check portion determining means for determining a check target portion and a presumed character string of the check target portion on the basis of the result in said word string candidate buffer; and check means for judging the possibility of existence of the presumed character string at the check portion regardless of the order of words in the character strings; wherein said check portion determining means determines as a check target portion a portion at which a part of a character string of a word string candidate in said word string candidate buffer is not obtained in said recognition result buffer, and determines the character string of the part as a presumed character string, and wherein if the number of segments in said recognition result buffer which correspond to the check target portion is coincident with the number of characters of the presumed character string, the individual character recognition is retried on the segment corresponding to the presumed character string by using individual character recognition means which is different from said cut-out recognition means, and existence of the presumed character string is admitted if the character candidate which is obtained as a result of the retrial is coincident with the presumed character string.