A method of an automatic adjustment of a carrier level is used for a system having a center device, a communication line extending from the center device, and at least one communication-terminal device connected to the communication line. The system conducts two-way communication of data signals on the communication line based on a predetermined multiplexing scheme. The method includes the steps of periodically transmitting a carrier-level-measurement message from the center device to the communication line for each high-frequency channel used for the two-way communication, the carrier-level-measurement message being sent via a first channel different from channels used for communication and control, transmitting a reply message in response to the carrier-level-measurement message from a communication-terminal device to the center device via a second channel different from the channels used for communication and control, transmitting a carrier-level-adjustment message from the center device to the communication line via the first channel, the carrier-level-adjustment message containing level-adjustment information based on a carrier level of the reply message received by the center device, and adjusting a carrier level of the communication-terminal device based on the level-adjustment information of the carrier-level-adjustment message received by the communication-terminal device.