In a light transmission system, each terminal equipment amplitude-modulates an optical main signal with a supervisory control signal of a proper frequency allocated to respective repeating installations. The repeating installations which have received the optical main signal demodulate the supervisory control signal from the optical main signal. The repeating installations detect that the supervisory control contains the own frequency component to judge it as a supervisory control signal addressed to the own installation from the terminal equipment. Alternatively, a branching installation is provided to extract the supervisory control signal before being coupled by an optical coupler in the branching installation, and amplitude-modulates the optical main signal at the output of the optical coupler with the supervisory control signal.