PURPOSE: To prevent unnecessary interruption of non-inspection terminal due to an inspection input to an inspection terminal by providing means for applying an inspection input after receiving an inspection confirming signal if the counterpart terminal halts at the time of inspection and there is no defective transmission.

CONSTITUTION: An AND circuit 11A receives a signal 101A which goes '1' upon receiving an inspection command at its terminal and a signal 102A which goes '1' upon receiving an establishment of inspection confirmation from a non-inspection terminal and produces an output signal 105A. An AND circuit 12A receives the signal 101A, a signal 104A which goes '1' when the non- inspection terminal A halts and a signal 103A which goes '1' when the transmission line between the terminals A and B is defective, and provides an output 106A to an ON delay timer 13A having time limit of T1 which then produces an output 107A. The output signals 105A, 107A are then fed to an OR circuit 14A which produces an inspection input applying signal 108A.