PURPOSE: To prevent data in a memory card destroyed by preventing erasing and writing on the memory card by providing a mechanism to prevent erroneous erasing and writing at the memory card, providing a detector to detect the state of the above prevention mechanism at a card reader, and transmitting the output of the detector to a central processing unit.

CONSTITUTION: When the rewriting of the data stored in the memory card 1 is performed, a shutter 8 is saved from a hole 3. When it is desired to protect or to inhibit the writing of the data stored in the memory card, the hole 3 is blocked by sliding the knob of the memory card. When the memory card 1 is inserted to the card reader 2, the light of a photocoupler 6 is shielded with the shutter 8 of the memory card 1. Information representing the above shielding is transmitted from the photocoupler 6 to the central processing unit, and the central processing unit judges that the rewrite is infeasible based on received information, and performs no erasing or writing operation. In such a way, the destruction of the data in the memory card 1 can be prevented.