PURPOSE: To discriminate the section where a fault is being generated easily with simple constitution by identifying a pattern of an alarm suppression signal so as to discriminate a faulty location of a high order group transmission line.

CONSTITUTION: Alarm suppression signal generating sections 14-16 generating an alarm suppression signal of a different pattern are provided respectively in the 1st and 2nd digital transmission terminal station 11, 12 and in a relay station 13. Then an alarm suppression signal with a different pattern is generated from the alarm suppression signal generating section 14 or 15 in the digital transmission terminal station connecting to the high order group transmission line having a fault in the high order group transmission lines 4, 5 and the alarm suppression signal generating section 16 in the relay station 13 and sent to low-order group transmission lines 8, 9. Since the alarm suppression signal is utilized in this way to discriminate the fault occurrence location of the digital transmission system, the section where a fault is being generated is discriminated easily with simple constitution.