PURPOSE: To suppress the increase of document capacity as compared with the number of modified characters when some modifications are performed for the characters prepared by the use of a Japanese word processor.

CONSTITUTION: A control means 1 makes a character string modification means 2 modify the character that a user desires. A character string modification means 2 passes the information on the modified character to character modification information control/generation means and makes the means prepares a character modification list. When the modification of the character is terminated and a document storage instruction is received, the control means 1 instructs a character modification information optimizing means 4 to compress character modification information. The character modification information optimizing means performs the compression of character modification information by taking out only displaced modification information based on a character modification list. When the compression is terminated, a data storage means 5 stores the record of only a Japanese character string and the record of compressed character modification information in a document file by dividing the records.