PURPOSE: To attain accurate fault location by receiving a transferred test frame, referencing connection position information when there is any node processor not receiving the frame, and switching a faulty location.

CONSTITUTION: Node processors 22-24 receiving a test frame identify the frame to be the test frame, then makes a sender address in the test frame into a destination address and sets its own node processor address information as the sender address and transfers the test frame to a common transmission line 3. A processor 11 receives the test frame transferred from the node processors 22-24 within prescribed period to check the sender address in the test frame. Then connection location information in the constitution information is referenced and the processor 11 locates whether a fault resides in a node processor not receiving the transferred test frame or in at transmission line 3 depending on the relation of the position of the node processor receiving the transferred test frame and the node processor not receiving the transferred test frame.