PURPOSE: To easily and surely perform battery exchange without destroying the content of a memory by exchanging the first battery after the power supply to the memory is stabilized by connecting the second battery or a substitute DC power supply to a connector after the system power supply is turned off at the time of performing the battery exchange.

CONSTITUTION: At the time of exchanging the first battery 4a, the second battery having a sufficient capacity is connected to a connector so as to stabilize the power supply to a data holding memory 1 after system power supply is turned off. After the power supply to the memory 1 is stabilized, the first battery 4a is removed and a new battery 4b having a sufficient capacity is mounted in place of the old battery 4a. When the second battery 5 is disconnected front the connector 3, the battery exchange is completed. Therefore, the battery exchange can be performed easily and surely without destroying the data in the memory.