PURPOSE: To simplify the circuit configuration of a signal transmission device by cutting off a low frequency component of a data signal, and synthesizing the resulted signal with a voice signal to transmit, then separating the received signal to reproduce the low frequency component, in case of superposing the data signal to the voice signal and transmitting them.

CONSTITUTION: A low frequency removing circuit 50 removes a low frequency component from a data signal inputted to an input terminal 10 and an analog adder 14 composes the low frequency component with a voice signal inputted to an input terminal 11 analogically. A branching filter 16 separates the synthesized signal transmitted through a transmission line 15 into a data signal from which the low frequency component is removed and the voice signal. The data signal is recovered to the original signal through a low frequency reproducing circuit 51 consisting of a clamp circuit and a comparator.