PURPOSE: To attain ease of fault check at fault of a relay line by constituting the titled system of a pattern generating collating means and a multiplex line concentrating means and providing a control means for applying multiplex control to a faulty circuit.

CONSTITUTION: If a fault takes place in a relay line 306, a modulator- demodulator 304 detects a line fault, transmits a line quality deterioration signal to a multiplex line concentrator 303 and gives it to a control circuit 110 via a level converting circuit 116. The circuit 110 supervises the line quality of a relay line and has a function of multiplex separation control, transmission/ reception of control information and input/output control of test pattern to which each low speed line data of transmission/reception circuits 105∼108 is to be multiplexed in relay lines 1, 2. In controlling the test pattern from a pattern generating circuit 121 that the test pattern from a pattern generating circuit 121 is transmitted to the relay line 1 via the circuit 110 with multiplexing, a multiplex line concentration circuit 109 multiplexes the test pattern and transmits the result to the device 304 via a transmission/reception circuit 111 and a level converting circuit 114. The test pattern is returned by the device 304, collated with the transmission pattern to locate the fault.