PURPOSE: To add a new line concentration module to an existing exchange without giving effect on a main body channel system and an exchange software, by performing autonomously management, selection and control of a channel.

CONSTITUTION: A module control circuit 5 executes periodically the loop monitor of a subscriber line accommodated in a line module 8, selects autonomously an outgoing channel at the detection of a call, its channel number is added and calling information is transmitted to a time division channel 11. On the other hand, the channel 11 transmits a terminal number of an incoming subscriber to the module 8 in case of incoming to the subscriber, and when the circuit 5 selects a channel, then the number is reported to the channel 11 and a ringing signal is transmitted to the incoming subscriber line. The channel 11 and a central processing unit 11 have only to process closing/releasing of a time division channel according to the channel number on a time division highway 6 and process the call independently of the line concentration switch constitution in the module 8.