PURPOSE: To attain a returning test at a relay line having a fault by providing an operation control section controlling that a test signal from one of low speed lines is connected to a line having a fault in relay lines.

CONSTITUTION: If a fault takes place in a relay line 106, a modulator- demodulator 102 detects a line fault, informs a line quality deteriorating signal 44 to a multiplex line concentrator 101 and the signal is inputted to a reception control section 43. The control section 43 applies line concentration to a low- speed line data inputted to a multiplex and separating section 39 to a normal relay line 108 and applies control to be outputted to a high-speed line transmission buffer 41. The reception data from the control section 43 is separated at each low speed line by the multiplex separating section 39 and outputs the data to low-speed line transmission/reception buffers 31∼38. A measuring device 104 is connected to the low-speed line 109 in this state, the test data from the line 109 is transmitted to the device 102 via a transmission buffer 40 under the control by the operation control section 42 to abolish the data given to the line 109 from the line 108 to the control section 43. The test data is returned by the device 102 and collated by the measuring device 104.