PURPOSE: To prevent wiretap, by stopping communication if the number of responses from other devices is more than the prescribed numbers, in a network system in common use of one channel.

CONSTITUTION: Each device 2 is provided with a counter 24, which is reset to 0 when a data is transmitted and is incremented by 1 through the check with an ID number added to the responses. If two or more responses are returned to the same data, it is discriminated as wiretap and the result is informed to the application program in the device or the user to stop communication.