An optical add/drop device comprising: first wavelength selection means and second wavelength selection means configured to select and output an optical signal of a predetermined wavelength from an input optical signal; first branching means configured to selectively output a first signal being an optical signal input from a first terminal station on a main route to the first wavelength selection means or the second wavelength selection means; second branching means configured to selectively output a second signal being an optical signal input from a second terminal station on a sub-route to the first wavelength selection means or the second wavelength selection means; and first output means configured to selectively output, to the second terminal station, one of an optical signal to be output from the first wavelength selection means, based on the first signal and the second signal, and an optical signal to be output from the second wavelength selection means, based on the first signal and the second signal, as a third signal; wherein the first branching means is configured to selectively output the first signal as a fourth signal or a fifth signal; the second branching means is configured to selectively output the second signal as a sixth signal or a seventh signal; the first wavelength selection means is configured to select, when the fourth signal and the sixth signal are input, a signal of a predetermined wavelength from the fourth signal and the sixth signal, and is further configured to generate and output an eighth signal; the second wavelength selection means is configured to select, when the fifth signal and the seventh signal are input, a signal of a predetermined wavelength from the fifth signal and the seventh signal, and is further configured to generate and output a ninth signal; and the first output means is configured to selectively output one of the eighth signal and the ninth signal, as the third signal; and wherein the optical add/drop device further comprises: third branching means configured to selectively output an eleventh signal being an optical signal input from a third terminal station on the main route, to the second wavelength selection means or a third wavelength selection means; and fourth branching means configured to selectively output a twelfth signal being an optical signal input from the second terminal station, to the second wavelength selection means or the third wavelength selection means; wherein the optical add/drop device includes second output means configured to selectively output, to the second terminal station, one of an optical signal to be output from the second wavelength selection means, based on the eleventh signal and the twelfth signal, and an optical signal to be output from the third wavelength selection means, based on the eleventh signal and the twelfth signal, as a thirteenth signal. first wavelength selection means and second wavelength selection means configured to select and output an optical signal of a predetermined wavelength from an input optical signal; first branching means configured to selectively output a first signal being an optical signal input from a first terminal station on a main route to the first wavelength selection means or the second wavelength selection means; second branching means configured to selectively output a second signal being an optical signal input from a second terminal station on a sub-route to the first wavelength selection means or the second wavelength selection means; and first output means configured to selectively output, to the second terminal station, one of an optical signal to be output from the first wavelength selection means, based on the first signal and the second signal, and an optical signal to be output from the second wavelength selection means, based on the first signal and the second signal, as a third signal; wherein the first branching means is configured to selectively output the first signal as a fourth signal or a fifth signal; the second branching means is configured to selectively output the second signal as a sixth signal or a seventh signal; the first wavelength selection means is configured to select, when the fourth signal and the sixth signal are input, a signal of a predetermined wavelength from the fourth signal and the sixth signal, and is further configured to generate and output an eighth signal; the second wavelength selection means is configured to select, when the fifth signal and the seventh signal are input, a signal of a predetermined wavelength from the fifth signal and the seventh signal, and is further configured to generate and output a ninth signal; and the first output means is configured to selectively output one of the eighth signal and the ninth signal, as the third signal; third branching means configured to selectively output an eleventh signal being an optical signal input from a third terminal station on the main route, to the second wavelength selection means or a third wavelength selection means; and fourth branching means configured to selectively output a twelfth signal being an optical signal input from the second terminal station, to the second wavelength selection means or the third wavelength selection means; wherein the optical add/drop device includes second output means configured to selectively output, to the second terminal station, one of an optical signal to be output from the second wavelength selection means, based on the eleventh signal and the twelfth signal, and an optical signal to be output from the third wavelength selection means, based on the eleventh signal and the twelfth signal, as a thirteenth signal.