PURPOSE: To shorten the leading time of a back-up system and also to simplify the change of data by loading programs and data stored in a ROM to a system at the time of the system leading and then writing the change data stored writably with the back-up of a battery into the system in accordance with a change address.

CONSTITUTION: A back-up device 2 is connected to an input/output bus 7 via an interface equal to that of an ordinary input/output device. Then the programs, the fixed data, the subscriber data obtained when a station is set up and the station data obtained when the station is set up are stored in a ROM 4 according to the same address as that of a main memory 1. A rewritable memory 5 backed up by the battery is provided to change such data as the subscriber data, etc., which are daily rewritten after the station is set up. Then the change address ADR and the changed data of the memory 1 are stored in the memory 5. When an input/output instruction given from a CPU is received by a control circuit 3 at the time of a system leading, the contents of the ROM 4 are transferred directly to the memory 1.