One new file and one old file are sequentially compared from the heads of the files on a predetermined data unit basis. If a data match is found, transfer information showing a position and a data length of the old file is output. If a data mismatch is found by the sequential comparison, the old file is searched in both directions from the data position where the mismatch had been found in the old file, thereby finding a portion which matches with the data of the new file. If the matching portion can be found, shift information up to the matching portion on the old file is output. If the matching portion cannot be found, new data is added to transfer replacement information showing a length of data of the mismatching portion of the old file at a position until the matching portion can be found, and the resultant transfer replacement information is output.