15. A document management method for managing a correction history of a document, the method comprising the steps of: accepting, by a document input section, input of the document which is original and storing, by the document input section, the document inputted in a document store section; correcting, by a document correction section, a character string of the document stored in the document store section in response to operation input, registering, by the document correction section, a correction history including a number of characters deleted by a character string deletion process and a correction history including a number of characters before replacement by a replacement process in the correction history table, and storing, by the document correction section, the corrected document in the document store section; and referring, by a correction history analysis section, to the correction history table, and making, by the correction history analysis section, a decision table including an opacity rate, the opacity rate obtained by dividing a total of the number of the characters deleted by the character string deletion process and the number of the characters before the replacement by the replacement process by a number of characters included in the original document.
15. A document management method for managing a correction history of a document, the method comprising the steps of: accepting, by a document input section, input of the document which is original and storing, by the document input section, the document inputted in a document store section; correcting, by a document correction section, a character string of the document stored in the document store section in response to operation input, registering, by the document correction section, a correction history including a number of characters deleted by a character string deletion process and a correction history including a number of characters before replacement by a replacement process in the correction history table, and storing, by the document correction section, the corrected document in the document store section; and referring, by a correction history analysis section, to the correction history table, and making, by the correction history analysis section, a decision table including an opacity rate, the opacity rate obtained by dividing a total of the number of the characters deleted by the character string deletion process and the number of the characters before the replacement by the replacement process by a number of characters included in the original document. 16. The document management method according to claim 15 , wherein: a correction history indicative of a character string deletion process, a character string addition process, and a character string replacement process performed on the document are registered in the correction history table by the document correction section; and if a character string is added at a place where a character string is deleted, it is recognized by the document correction section that character string replacement is performed, and a correction history is registered in the correction history table.