Characteristic amounts in each small region of audio signals transmitted in the working system and the standby system are extracted by characteristic amount calculators 6-1, 6-2. A characteristic amount comparator 7 compares the characteristic amounts and judges occurrence of a fault. Characteristic amount difference calculators 9-1, 9-2, ∥D∥ comparator 10, and faulty system judging unit 11 judges the system having a fault. Majority decision processor 12 and significance judging unit 13 enhance the reliability of the judgment. Delay difference of audio signals between systems is roughly detected by sub-sampling audio signals of two systems and comparing them, and then accurately detected without sub-sampling. Delay difference between audio signals is adjusted by the detected delay difference.