An optical branching unit for an optical communication system, the optical communication system including a plurality of the optical branching unit and paths for a WDM signal between the optical branching units being connected in such a way that more than one path between the optical branching units exist, the optical branching unit comprising: optical add-drop means arranged to output an input WDM signal to any of a plurality of paths on a per-wavelength basis, an optical signal contained in the WDM signal being passed through at least one section, the section being external to the optical branching unit and being coupled with the optical branching unit; light wavelength detecting means arranged to detect a wavelength of the input WDM signal; a wavelength database arranged to store, on a per-wavelength basis, information about the section through which the optical signal contained in the WDM signal passes, the information being stored during construction of the optical communication system beforehand; and control means, the control means being coupled to the optical add-drop means, the light wavelength detecting means and the wavelength database, arranged to: store an alternative setting of the optical add-drop means for bypassing a particular section of the at least one section in association with a location of the particular section; and change a setting of the optical add-drop means in such a way that a path of an optical signal of a wavelength passing through the particular one of the at least one section bypasses the particular section based on the stored alternative setting of the optical add-drop means without performing communication about the particular one of the sections with other optical branching units when every wavelength stored in the wavelength database is not detected in the particular section by the light wavelength detecting means. optical add-drop means arranged to output an input WDM signal to any of a plurality of paths on a per-wavelength basis, an optical signal contained in the WDM signal being passed through at least one section, the section being external to the optical branching unit and being coupled with the optical branching unit; light wavelength detecting means arranged to detect a wavelength of the input WDM signal; a wavelength database arranged to store, on a per-wavelength basis, information about the section through which the optical signal contained in the WDM signal passes, the information being stored during construction of the optical communication system beforehand; and control means, the control means being coupled to the optical add-drop means, the light wavelength detecting means and the wavelength database, arranged to: store an alternative setting of the optical add-drop means for bypassing a particular section of the at least one section in association with a location of the particular section; and change a setting of the optical add-drop means in such a way that a path of an optical signal of a wavelength passing through the particular one of the at least one section bypasses the particular section based on the stored alternative setting of the optical add-drop means without performing communication about the particular one of the sections with other optical branching units when every wavelength stored in the wavelength database is not detected in the particular section by the light wavelength detecting means. store an alternative setting of the optical add-drop means for bypassing a particular section of the at least one section in association with a location of the particular section; and change a setting of the optical add-drop means in such a way that a path of an optical signal of a wavelength passing through the particular one of the at least one section bypasses the particular section based on the stored alternative setting of the optical add-drop means without performing communication about the particular one of the sections with other optical branching units when every wavelength stored in the wavelength database is not detected in the particular section by the light wavelength detecting means.