PURPOSE: To detect an unrevised line having the same contents as a program line which is deleted or changed in a revised program at the time of preparing revised history.

CONSTITUTION: A revised history preparing instruction is inputted from an I/O part 1 and a revision matching means 21 reads out programs concerned respectively from a current program storing part 31 and a revised program storing part 32, partitions adding parts, deleting parts and changing parts in those programs and stores these partitioned parts in an intermediate data storing part 33 as intermediate data. A revision display means 22 writes partitions such as addition and changes and a current date on a comment column in the corresponding line of the revised program based upon the intermediate data and an unrevised part detecting means 23 detects an unrevised line in the revised program based upon the intermediate data, adds the detected line to the intermediate data and stores the intermediate data in the storing part 33. An edition and output part 24 reads out the contents stored in the storing part 33 based upon a revised history output instruction outputted from the I/O part 1 and outputs the revised history edited by a prescribed format to a printer 4.