Transmission and communication of control information through a common signal transmission line are performed with a high transmission efficiency by the use of a control data signal including a content code signal portion and a sub-data signal portion but no address signal portion. The content code signal portion and the sub-data signal portion are representative of a brief content and a supplemental content of control forming in combination control data in the control data signal, respectively. A station with which an equipment is connected and which is coupled with the common transmission line contains at least one content code data so that when the content code signal portion of the control data signal is concurrent with one of the at least one content code data the equipment takes the control data signal for performing the control defined by the control data formed by content code and the sub-data signal portions of the received signal. The equipment is also capable of transmitting a control data signal similar to the above-mentioned one to the common transmission line.