PURPOSE: To allow the laser frequency of an upper node to coincide with that of a lower node in an optical branching/inserting transmission circuit.

CONSTITUTION: An optical branching system includes a local oscillation laser 10 for a lower node, the 1st optical heterodyne receiver 5 for executing optical heterodyne reception by an output from the laser 10 and an optical signal selected by an optical branching filter 2, a detector 7 for obtaining a branched receiving signal from a beat signal received by the receiver 5 and the 1st frequency discriminator 11 for inputting the beat signal and outputting an optical frequency error signal between an upper node transmission laser and the laser 10. A light inserting system includes a lower node transmission laser 16 for generating an inserting optical signal, the 2nd light heterodyne receiver 12 for executing optical heterodyne reception by the outputs of the transmission laser 16 and the local oscillation laser and the 2nd frequency discriminator 14 for receiving a beat signal from the receiver 12 and outputting an error signal between the optical frequency of the laser 16 and that of the laser 10.