PURPOSE: To insert voice signals of all stations desiring the insertion without overload by multiplying an insertion signal with a coefficient being ≤1 and adding the result to the voice signal of a main transmission line.

CONSTITUTION: A voice signal inputted to an input terminal 102 becomes the 2nd input of an adder 1 via a coefficient device 2. Thus, the sum of the voice signal and the insertion signal of the main transmission line is outputted from the adder 1 and this signal is outputted to an output terminal 103. The coefficient device 2 gives a degree of loss to the insertion signal so as to prevent an adder 3 from being overloaded. Thus, the insertion and transmission of the voice signal are attained without causing overload from an optional station.