PURPOSE: To obtain a reliable method by laying plural optical transmission lines at their changeover part at any time and using an extra stand-by optical transmission line side in case of trouble fault or trouble rearrangement.

CONSTITUTION: When there are three subscribers, for example, a branch optical trans mission line L1 is used as a trunk line, three waveguide chips 2 of the type of a branch waveguide 3b are fitted to the optical transmission line L1, and they are connected to subscriber-side transmitter/receivers 9... as external communication means through an optical fiber 10a, a subscriber terminal optical branching and coupling device 8, and an optical fiber 11. Consequently, a normal optical communication is carried on, but if fault or trouble rearrangement is required on the side of the branch optical transmission line L1, this branch optical transmission line L1 needs to be disconnected temporarily. At this time, another branch optical transmission line L2 is used as a stand-by optical transmission line and three waveguide chips 2 of the type of the branch waveguide 3b are fitted to the side of the transmission line L2. Then, those chips 2 and a subscriber terminal optical branching and coupling device 8 are connected mutually by an optical fiber 10b. Consequently, the subscriber-side transmitter/receiver 9 are connected to the trunk optical transmission line L0.