PURPOSE: To eliminate the generation of an error of replacement in case many files are frequently replaced, by adding the latest display to the latest file and a replaced display to a replaced file, respectively.

CONSTITUTION: A replacing means UPD first reads a display region P1 of a file F1 before replacement through a read-in part R0 and confirms the presence of the latest display. When no latest display exists, the replacing process is not executed. When the latest display exists, the data of the file F1 is read into a buffer BUF1 through a reading part R1. Then the replaced data CR is read into a buffer BUF2 through a data reading part R2. The data to be replaced in the buffer BUF1 is replaced by the replaced data in the BUF2 and through a replacing part UP, and the replaced data is delivered to a buffer BUF3. A read- out part W1 reads out the replaced data of the BUF3 into a new file F2. Then a write-out part W0 writes a replaced display into the region P1 of the old file F1 and the latest display to a display region P2 of the file F2 respectively when the write-out is over.