PURPOSE: To transmit voice of all conference participants at a uniform level by inserting a variable gain circuit to a transmission line and a reception line between an exchange trunk and a conference use bridge.

CONSTITUTION: Variable gain circuits As1∼Asn for transmission and variable gain circuits AR1∼ARn for reception controlled by a talker voice signal are provided to input/output transmission lines and a reception line of the conference use bridge B. Then each talker voice signal is controlled automatically so that the average level of the signal at an output point from the conference use bridge B to the exchange trunk C is made uniform. Thus, the loss of transmission lines L1∼LN from subscriber telephone sets to the conference use exchange trunk C being in variation to various values is adjusted to a prescribed value, the result is added and distributed to the conference use bridge B so that the voice of all the conference participants is transmitted to other conference participants at a uniform level.