PURPOSE: To prevent missing of stored content positively by automatically locking a loaded memory card until replacement of backup battery is finished when the voltage of backup battery drops below a predetermined level.

CONSTITUTION: When an undervoltage detecting circuit 7 detects voltage drop of a backup battery Ba below a predetermined level under a state where a memory card 2 is loaded to an electronic appliance 1, an automatic lock lever 15 rotates through a solenoid driving circuit 8 to lock a locking click 9. The lever 15 is held in locked position until loading of a new battery Ba is finished and when the battery Ba is replaced, a non-volatile memory 3 in the card 2 is fed with power from a driving power supply 6 of the appliance 1. According to the constitution, missing of memory content of the card 2 due to erroneous operation can be prevented positively when the backup battery is replaced.