PURPOSE: To detect a fault in a half disconnected state in a transmission line without fail by transmitting the test frame of a variable length without possessing any token.

CONSTITUTION: A beacon frame (ring error announce frame) 23 is transmitted from an arbitrary configuration controller 22, and all the stations in a ring are turned to a frame repeat (reproduction repeating) state. Then, the configuration controller 22 transmitting the beacon frame 23 confirms the repeat state. Next, the configuration controller 22 transmits a test frame (variable length long frame) 24 addressed to the own station without possessing any token. The configuration controller 22 checks it according to the contents of the received test frame whether this test frame returns after turning around in the ring or not, and detects the presence/absence of the fault. Thus, even when the fault in the half disconnected state to pass the beacon frame 23 is generated in a token ring LAN, this fault is detected without fail.