PURPOSE: To make the attendance to an unattended station for the test toward incoming direction unnecessary, by transmitting the pulse signal in a monitor frequency component fi+1 at the unattended station to an incoming transmission line, according to the pulse signal in a monitor frequency component fi transmitted from an attended station.

CONSTITUTION: An unattended station 7 is provided with a monitor frequency converter SVFC7. When this SVFC7 inputs the pulse signal in a monitor frequency component fi (e.g., the same frequency as that assigned to a filter 3a), the pulse signal in a monitor frequency component fi+1 (e.g., the same frequency as that assigned to a filter 3b) is outputted. Thus, when the pulse signal in the component fi is transmitted from an attended station, it returns to the attended station via a relay device 1a, a filter 3a, and a monitor line 6 for level measurement. Further, when the pulse signal in the component fi+1 is outputted from the SVFC7 according to the pulse signal in the component fi, it returns to the attended station via a relay device 2b, a filter 3b and the monitor line 6 for level measurement.