PURPOSE: To obtain a qualification system with high reliability by allowing a central device to distribute only an open key to users in communication between the users and allowing the user to use a secret key between the users generated based on the open key to cipher data for communication.

CONSTITUTION: The user 1 transmits a distribution request RQ2 of the open key KP2 and the own identification code ID1 in the form of plain message in the communication between the users 1 and 2. The central device 3 uses the key generated by the user 1 only to cipher the code ID1 and the key KP2 and transmits a ciphered message EZ13 to the user 1. When the code ID1 obtained by using the ciphered message is coincident with the own code, the user 1 qualifies the ciphered message EZ13 to acquire the key KP2 and transmits the request telegraphic message RQ12 and the code ID1 in the form of a plain mes sage to the user 2. Similarly, the user 2 requests the open key KP1 of the user 1, uses the own identification code ID2 to ensure the key KP1. The user 2 transmits the ciphered message of the ID1, ID2 using a secret key KS2 formed by using the said key to the user 1. The user 1 decodes it, recognizes the coinci dence with the own ID1 and transmits the ciphered message where the logical value of the ID2 is inverted to the user 2.