PURPOSE: To automatically investigate the address setting of a communication slot, by detecting a discriminating signal in each communication slot of a frame when the frame arrives at a reference station.

CONSTITUTION: When a slot assignment is performed by using a node A as a reference node, the node A sends out a synchronizing slot after a synchronizing signal is put in the slot and a communication slot CH3 after a discriminating signal XA is put in the slot CH3. At a node B the position of its own slot is known by detecting the synchronizing signal and a communication slot CH1 is sent out after a discriminating signal XB is put in the slot CH1. At nodes C and D communication slots CH4 and CH2 are sent out after discriminating signales XC and XD are put in the slots CH4 and CH2. When this frame arrives at the node A, it can be known that no double address setting is made in the address setting of the coummunication slot by detecting the content of each slot of the frame at the node A.