PURPOSE: To reduce the cost for the investment to an initial equipment for the introduction of digital service, by availing an existing telephone use analog exchange as a line concentrator to digital terminals.

CONSTITUTION: An analog telephone set 1, digital terminals 2, and a line terminator 3 are connected to a subscriber line 5 via a terminal switching device 4 and the line 5 is connected to an analog exchanger 6. The exchange 6 is connected to a digital transmission line 8 via an interface 7 for analog exchange and to a digital exchange 9 via the transmission line 8. The interface 7 is provided with an encoder 13 encoding analog signals, an intra-office line terminator 14 reproducing digital signals, a detector 18 detecting a switching instruction signal and a cut-off signal, and a switching 12 switchingly connecting the encoder 13 or the terminator 14 to an exchange 16 with information of the detector 18, for the mutual connection of the exchanger 6 and 9 through the exchange between analog and digital signals. Thus, the exchange 6 acts like a line concentrator for the terminal 2.