PURPOSE: To prevent the power for backup from being supplied from a battery for backup at the time of inserting this battery to a device whose power source has never been turned on.

CONSTITUTION: If the power source of the device on which a memory 4 is mounted has never been turned on and a battery 3 is set to this device, a point (a) of a flip flop 5 keeps the high level to keep an FET turned off hereafter. Therefore, the backup power is not supplied from the battery 3 to the memory 4. When the power source of the device is turned on thereafter to bring about the state that a microcomputer 7 can set the memory to the low power consumption mode, a signal 100 is set to the high level to set the point (a) of the flip flop 5 to the low level, and the FET 6 is kept turned on hereafter; and therefore, the backup power is supplied from the battery 3 to the memory 4 through an FET 6 when the power source of the device is turned off.