PURPOSE: To monitor and inform fault by an ISDN user network interface by assigning plural terminals to a prescribed frame number position of S bits which a network terminating equipment sends and sending prescribed alarm information informing the abnormality and assigning plural terminals of prescribed Q positions of FA bits.

CONSTITUTION: Terminals 1 to 8 send binary code '0' as bits Q1 to Q4 before multiframe synchronism acquisition and send binary code '1' for unused state or normalcy or binary code '0' for abnormality as alarm information after synchronism acquisition. That is, either of terminals 1 and 2 sends binary code '0' as the bit Q1 when detecting abnormality or synchronism trouble by discrimination of the pattern sent from a network terminating equipment 9. In the same manner, terminals 3, 4, 5, 6, 7, and 8 send binary code '0' as bits Q2, Q3, and Q4 respectively at the time of abnormality or fault.