To properly maintain the safety of the whole of a system by ciphering proper identification information with the first and the second information, and selecting and ciphering a part of the information with the third information for generating an access key.

An ID number inherent to a card is ciphered, using the preset fixed data (initial value of identification number) and a suitable certification number. The ciphered initial information so generated is further ciphered with a system key. Thereafter, an expanded key containing information longer than the information required for an access key is generated from the information ciphered with the system key through a ciphering algorithm. This ciphering algorithm does not need to be decoded, and a random number generation function may, for example, be used. A part of the expanded key so generated is selected and a temporary key is generated. This temporary key is ciphered with a terminal number assigned to each terminal device, thereby generating the access key.