Printed circuit boards 11 to 11n are connected in a star-like configuration with a single packet processing IC 42, connected to a CPU 41, at its center, each printed circuit board being connected to the packet processing IC by a high-speed supervisory control line 21 having a sufficient transmission capacity to transfer therethrough transparent information and alarm transfer information as well as information from the central processing unit in packet form, and the transparent information and the alarm transfer information are communicated between the printed circuit boards via the high-speed supervisory control line and via the packet processing IC, with provisions made for the packet processing IC to detect a destination from the packet information received from the originating printed circuit board and to transmit the packet information to the terminating printed circuit board. Further, provisions are made to transmit packetized cells to the terminating printed circuit board in accordance with time priority, thereby achieving a transmission line terminating apparatus with a reduced number of wiring lines.