PURPOSE: To decrease the number of connecting core wire by providing a CODEC to each key telephone device in accordance with the number of stored trunk line of a main device and transmitting both audio signal information and control signal information through a common highway.

CONSTITUTION: Plural key telephones TEL1∼TELn are connected to a main device 1 in parallel to each other via a common highway. At the same time, the control information on the telephones TEL1∼TELn as well as the device 1 are multiplexed in time division. Then CODEC8 and 16 are provided to the device 1 in correspondence to the number of store circuit and to the telephones TEL1∼ TELn to convert each audio signal into a pulse signal and vice versa respectively. Thus each audio signal information is transmitted into the same time slot as the control signal information through a common high way (b).