PURPOSE: To make the continuity test of a talking line of a common line signal system through the use of the individual signal of the individual signal system by providing a continuity confirming signal return means returning a continuity confirming signal in terms of the individual signal to a line circuit for the individual signal system to the system.

CONSTITUTION: Since a signal transmitter-receiver 17 of a caller station 1 monitors a state of a signal bit received by a BWT 14, the change of the signal bit to A=1, B=1 is detected and the result is sent to a continuity test processing section 15 of a call processing processor 12 via an SP bus 18. The continuity test processing section 15 confirms the return of the continuity confirming signal based thereon and sends a continuity test success signal (COT) to a called station 2 via a common line signal device 13 and controls a signal transmitter-receiver 27 to stop the continuity test signal. A continuity confirming signal return section 25 of the called station 2 receiving the COT controls the signal transmitter-receiver 27 to stop the continuity confirming signal.