In order to provide a submarine optical transmission system that utilizes multiple wavelength bands, the submarine branching apparatus is provided with: a first demultiplexing part for demultiplexing a wavelength-multiplex optical signal input from a first terminal station into a first wavelength-multiplex optical signal and a second wavelength-multiplex optical signal; an optical add-drop part for outputting at least a third wavelength-multiplex optical signal included in the first wavelength-multiplex optical signal to a second terminal station, and for outputting a fifth wavelength-multiplex optical signal by multiplexing at least a fourth wavelength-multiplex optical signal included in the first wavelength-multiplex optical signal with a wavelength-multiplex optical signal input from the second terminal station; and a first multiplex part for multiplex the second wavelength-multiplex optical signal with the fifth wavelength-multiplex optical signal input from the optical add-drop part and outputting the resulting signal to a third terminal station.