an optical signal return unit, including one or more processors, connected to a representative node with an active/auxiliary configuration configured to send and receive a burst optical signal and send a continuous-wave clamp beam with a wavelength different from a wavelength of the burst optical signal, via two optical transmission lines with a double-ring configuration configured to perform optical transmission in mutually opposite directions, in a manner relaying the burst optical signal and a continuous optical signal of the clamp beam, the optical signal return unit being connected across the two optical transmission lines in each of a plurality of nodes that sends and receives the burst optical signal; and

a detection unit, including one or more processors, configured to, when an input optical signal via a first optical transmission line of the two optical transmission lines has not been detected for a predetermined time or longer, output a disconnection signal of the first optical transmission line to the optical signal return unit, wherein

only when there is an input of the disconnection signal, the optical signal return unit is configured to return to the first optical transmission line only a clamp beam that has been sent from the representative node via another optical transmission line in a direction opposite to the input optical signal via the first optical transmission line.