PURPOSE: To improve the talking voice quality by providing an automatic gain controller and an echo canceler between the central office line trunk and private line trunk of a private branch exchange PBX.

CONSTITUTION: An echo canceler 6 erases the echoes which are produced when the voice signals sent from a private line 2 are returned by a 2/4-wire converting circuit 8 of a central office line trunk 4 and a 2/4-wire converting circuit 10 included in an office exchange of a public network 1. An automatic gain controller 7 is included in a channel reaching to the line 2 from the network 1 and keeps a fixed talking level. That is, the reduced talking level is compensated by the controller 7 for the voice signals sent from the network 1. Thus the compensated voice signals are sent to the line 2. Furthermore the voice signals sent to the network 1 from the line 2 are erased by the canceler 6 even though these signals are returned by both circuits 8 and 10.