PURPOSE: To allow the voice compression transmitter to transmit a signal subject to voice coding at a low bit rate and a demodulated signal for MODEM or a facsimile equipment or the like independently of models of exchanges in an in-enterprise network with respect to provided between an exchange and a relay line through which a code signal at a low speed resulting from a compressed signal at a usual voice coding speed is sent.

CONSTITUTION: The transmitter is provided with a frame addition section 2 inserting a specific frame synchronizing signal to an input signal from a relay line to an exchange 11 and with a frame detection section 3 detecting the specific frame synchronizing signal from an output signal to the relay line in pairs with the input signal, and also with switches 4a-4d implementing the control such that a low speed code signal is transmitted to a relay exchange and a relay line when the frame detection section 3 detects the synchronizing signal, and a decoded voice signal is transmitted to the relay exchange and a compressed low speed code signal is transmitted to the relay line when the detection section 3 does not detect the synchronizing signal.