PURPOSE: To ensure securecy of information by cyphering secret information by data cryptographic key and recording and transferring the data cryptographic key cyphered by means of cryptographic key means.

CONSTITUTION: A terminal security unit 15 as a kind of cyphering devices takes a form of a black box and a proper cryptographic key TK whose contents are automatically extinguished when unsealing is done e.g. by illegal reverse engineering and aquired only by a user is sealed. The cryptographic key TK itself has not a function for directly cyphering plain text information (X), plays a role of a kind of master keys, cyphers a data cryptographic key DEX and is provided with the function of further cyphering the 'key' on algorism for cyphering processing of the plain text information. The data cryptographic key means itself is cyphered and freely stored or transferred.