PURPOSE: To prevent the data stored on a storage medium from being used illegally by storing encoded information or information indicating a method of decoding on the storage medium, and generating an unlocking signal in conformity with the information on the storage medium and a card.

CONSTITUTION: The memory M in the card stores the 1st decoding key in addresses A1WA3 and the 2nd decoding key in addresses B1WB3. A decoder DEC decodes a code and decodes encoded data (d) to obtain data d1 or d2 when the data d1 or d2 encoded and stored in the memory M is supplied. A comparator COM, on the other hand, compares decode processed data (d) from the decoder DEC with data (d) read out of the memory M and outputs a comparative coincidence or dissidence signal.