PURPOSE: To back up an IC memory file at the time of service interruption by controlling data writing in the 1st and 2nd auxiliary memory devices by using writing mode specifying information.

CONSTITUTION: An IC memory file 504, status information ML, MS of a magnetic disc memory device 505 and the writing mode specifying information WM indicating whether data are to be written in both the file 504 and device 505 or only in the file 504 are stored in a file control block 101. When the status information ML is '1', the master unit is active, and the information SL is '1', the slave unit is active. When the information WM is '0', data are written only in the master unit, and in case of '1', data are written both the master and slave units.