PURPOSE: To improve the reliability of loopback diagnosis corresponding to a line or the like by applying diagnosis of clock input prior to the start of loopback diagnosis corresponding to line.

CONSTITUTION: When the system enters the operating state, a detection signal corresponding to a line is transferred to a register 10 from a clock detection circuit 81 of a MODEM interface device 4 and set corresponding to the line. When a loopback diagnosis corresponding to the line is started by an information transmission reception controller 2, the set information corresponding to the line of the register 10 is referenced. When the set information represents the presence of the clock, the loopback diagnosis corresponding to the line similar to a conventional system is entered. Thus, even when the diagnosis is started without informing the presence of MODEM mount to the line corresponding loopback diagnosis system, diagnosis of a faulty line in mistake is not caused.