PURPOSE: To disperse the danger of ciphering key leakage by preserving a ciphering key while being separated into a generating means and a parameter.

CONSTITUTION: In inputting a parameter from a parameter input device 2, the parameter is stored in a parameter storage memory 7 in a ciphering key generating device 3 via a parameter input device adaptor 4 and a ciphering key generating device input/output adaptor 5, a microprocessor 6 in the device 3 executes a ciphering key generation program 8 to generate the ciphering key. The generated ciphering key is stored in a ciphering key storage memory 9 in a confidentiality protecting device 1 via a ciphering key generation input/ output adaptor 5. A ciphering processing device 10 applies ciphering/decoding to the content of a transmission/reception buffer, sends the ciphered key to an opposite party via a line and applies decoding to the reception data. thus, the confidentiality of the ciphering key is improved.