A submarine branching device comprising: branching means configured to branch a wavelength multiplexed optical signal input from a first terminal station and outputting a first branch signal and a second branch signal; optical add/drop means configured to output at least a first wavelength-multiplexed optical signal included in the first branch signal to a second terminal station, and to multiplex at least a second wavelength-multiplexed optical signal included in the first branch signal and a wavelength-multiplexed optical signal input from the second terminal station and to output at least a third wavelength-multiplexed optical signal; multiplexing means comprising wavelength selection means (96,97) configured to multiplex a fourth wavelength-multiplexed optical signal included in the second branch signal and the third wavelength-multiplexed optical signal in normal operation or a fifth-multiplexed optical signal included in the first branch signal in case of failure and to output the multiplexed signal to a third terminal station; first switching means configured to output first branch light input from the branching means to the optical add/drop means or the second switching means second switching means configured to output a third wavelength-multiplexed optical signal input from the optical add/drop means or the first switching means to the multiplexing means; and failure detection means configured to detect a failure of a transmission line between the second terminal station and the submarine branching device, wherein, in response to detecting a failure by the failure detection means, the first switching means is configured to switch a signal path from the optical add/drop means to the second switching means and to output the first branch signal to the second switching means, and the second switching means is configured to switch a signal path from the optical add/drop means to the first switching means and outputting the first branch signal to the multiplexing means. branching means configured to branch a wavelength multiplexed optical signal input from a first terminal station and outputting a first branch signal and a second branch signal; optical add/drop means configured to output at least a first wavelength-multiplexed optical signal included in the first branch signal to a second terminal station, and to multiplex at least a second wavelength-multiplexed optical signal included in the first branch signal and a wavelength-multiplexed optical signal input from the second terminal station and to output at least a third wavelength-multiplexed optical signal; multiplexing means comprising wavelength selection means (96,97) configured to multiplex a fourth wavelength-multiplexed optical signal included in the second branch signal and the third wavelength-multiplexed optical signal in normal operation or a fifth-multiplexed optical signal included in the first branch signal in case of failure and to output the multiplexed signal to a third terminal station; first switching means configured to output first branch light input from the branching means to the optical add/drop means or the second switching means second switching means configured to output a third wavelength-multiplexed optical signal input from the optical add/drop means or the first switching means to the multiplexing means; and failure detection means configured to detect a failure of a transmission line between the second terminal station and the submarine branching device, wherein, in response to detecting a failure by the failure detection means, the first switching means is configured to switch a signal path from the optical add/drop means to the second switching means and to output the first branch signal to the second switching means, and the second switching means is configured to switch a signal path from the optical add/drop means to the first switching means and outputting the first branch signal to the multiplexing means.