PURPOSE: To prevent a disguised malfeasance by providing first and second random value generating means for the first device, providing the third random value generating means, which is the same as the second random value generating means, for the second device, and comparing the output of the second random value generating means and that of the third random value generating means.

CONSTITUTION: A random value is outputted from the second random value generating mans 8 based on the output of the first random value generating means 5 of the first device 1 to successively output a long cycle data string. The output of the means 5 is also supplied to the third random value generating means 11 to execute the same operation processing as that of the means 8 of the second device 2. Further, the respective random values outputted by the means 8 and 11 are compared in a comparing means 14, and when the respective random values coincide with each other, the propriety of the device 2 is certified. In the above-mentioned composition the communication between the first and the second devices can be executed using the random values, and disguised malfeasance can be prevented even when the contents of the communication are tapped.