As a prior art, a correction method and verification method of the validity of the correction used in a paper document will firstly be described. 
Conventionally, as a representative correction method for use in performing correction for a paper document, there is known a method as shown in FIG.  48.  That is, the method includes: crossing out characters to be corrected with a double line and entering proper characters in the space immediately above the correction (P1), putting signatures (initials) of both parties thereto (P2).  Note that, in drawings, hereinafter, where the correction is made with a seal (name in a circle), this means the above correction method, that is, it corresponds to putting signatures of both parties thereto. 
It is possible to guarantee/confirm the following points by conducting the above P1 and P2 in the conventional correction method for a paper document. 
(1) Capable of easily confirming/identifying a corrected part, as well as capable of confirming that any other part than the corrected part includes no deliberate or negligent change. 
(2) Capable of easily confirming/identifying corrected area. 
(3) Capable of easily confirming a person who has made the correction. 
(4) Capable of confirming whether the corrected part is a correctable part. 
(5) Capable of confirming the original before correction. 
(6) Capable of making a correction and verifying the correction according to a policy (control information) related to corrections. 
Similarly, it is possible to guarantee/confirm the following points in the conventional correction method for a paper document with carbon-copied paper used for an insurance application form, transportation order form, or the like. 
(7) Capable of partially hiding the document, as well as confirming that any other part than the hidden part includes no change. 
(8) Capable of confirming the identity between a base paper and carbon-copied paper by comparing handwritings on respective papers. 
(9) Capable of detecting falsification of document by separately storing the base paper and carbon-copied paper. 
(10) Capable of performing a third-party certification related to the content of the document, even if being brought in court case, according to (9). 
(11) The base paper and carbon-copied paper can be distributed as needed.  In some cases, they can independently be distributed of each other. 
As described above, a correction method and a method of certificating the validity of the correction in a paper document are excellent in various points.  Meanwhile, along with recent advancement in IT technology, a technique that handles digital data (digital information) in place of the above paper document in view of convenience of data handling and data storage has been proposed (refer to, e.g.,    Patent Document 1, 2 and 3 and Non-Patent Document 1 and 2 for example)    Patent Document 1: Jpn.  Pat.  Appln.  Laid-Open Publication No.  2000-285024    Patent Document 2: Jpn.  Pat.  Appln.  Laid-Open Publication No.  2001-117820    Patent Document 3: Jpn.  Pat.  Appln.  Laid-Open Publication No.  2003-114884    Non-Patent Document 1: Paper of Information Processing Society of Japan/Computer Security Group (CSEC) “Digital document sanitizing problem (2003/7/17) (2003-CSEC-22-009)”    Non-Patent Document 2: Paper of SCIS2004 “A Digital Document Sanitizing Scheme with Disclosure Condition Control”).