A signal transmission system for transmitting a first signal and a second signal superposed thereupon through a common signal transmission line composed of two wires. A plurality of terminal units are connected through the common signal transmission line to a central unit and are connected in parallel with each other across the two wires of the signal transmission line. At least one of the terminal units transmits the first signal in the form of a level signal; and at least one of the terminal units transmits or receive the second signal in the form of a superposed signal on the level signal. The central unit transmits and receives the superposed signal as well as receives the level signal for receiving data or information respectively from the above terminal units. Accordingly, both the first and second signals of different types and therefore carrying different information can be transmitted between the central unit and the respective terminal units through the common signal transmission line.