PURPOSE: To transmit information into an optional time slot of a PCM communication line without changing an internal telephone exchange body from a terminal equipment at all, by providing required devices such as an information inserting/extracting circuit and an information sampling device between the PCM communication line and the terminal equipment.

CONSTITUTION: Under the control of a processing device 15, information of a time slot of a PCM communication line 1 where information should be inserted is sent through the route of a communication controller 14, information transmitting devices 13 and 12, and an information inserting/extracting circuit 9 to set a timing for an information sampling device 10 and an information inserting circuit 11. Information transmitted from the communication line 1 is received by a terminal equipment 2, and information of the time slot set by the device 15 is sampled by the device 10 and is transmitted to the device 15. Meanwhile, time slot information is transmitted to another transmission line through terminals equipments 3 and 4, a multiplexer 5, and a digital switch 7. Information to be transmitted from the device 15 is put into the set time slot of the communication line 1 and is transmitted through devices 14, 13, and 12, the circuit 11, and the device 2.