PURPOSE: To check the capacity of a backup battery incorporated in a memory card for data preservation in plural levels without attaching a dedicated terminal additionally on the memory card.

CONSTITUTION: The voltage of the backup battery 12 is monitored based on two reference levels by a power source control circuit 8, thereby, first and second check signals can be obtained. The check signal on one side is read by an electronic still camera 20 from a power source data terminal (Vcc) allocated for exclusive use, and the check signal on the other side is sent to the electronic still camera 20 by utilizing a data terminal at a bit position not used in data communication out of the data terminals (D0-D7) of eight bits. The battery check circuit 27 of the electronic still camera 20 displays the voltage level of the backup battery 12 in plural stages by those first and second check, signals.