PURPOSE: To attain automatic and sure insertion by comparing a voice level of an insertion signal with a level of a voice signal of a main transmission line and inserting the signal when the voice level of the insertion signal exceeds the voice level of the main transmission line.

CONSTITUTION: A 1st signal level detector detecting the signal level of the voice signal of the line to be inserted and a 2nd signal level detector detecting the signal level of the insertion signal are provided. Then outputs of the detectors 1, 2 are compared and when the voice level of the insertion signal exceeds the voice signal level of the main transmission line, a control signal is outputted so as to switch a comparator 4 to the position of the insertion signal.