PURPOSE: To monitor the quality of a current line on the reception side by detecting coincidence/noncoincidence between error detection data sent from the transmission side and that on the reception side at the time of the non- speech state.

CONSTITUTION: At the time of non-speech, dial pulses are not changed equally to the transmission side of a voice multiplexer 1, and the non-speech state is detected by a time measurer 64 and an AND circuit 65 when the '1' (open) state continues for a certain time or longer. Then, the result of coincidence/ noncoincidence between a PN pattern as error detection data from a PN pattern generator (PTN) 21 on the transmission side and a PN pattern from a PTN 61 on the reception side is outputted from an AND circuit 66. Thus, the quality of the transmission line is checked on the reception side.