In order to allow for a flexible network configuration and establish an alternative path without performing communication between optical branching units in the event of a failure on a path in an optical communication system to which a plurality of optical branching units are connected, the optical branching unit includes optical add-drop unit for outputting an input WDM signal to any of a plurality of paths on a per-wavelength basis, light wavelength detecting unit for detecting a wavelength of the input WDM signal, a wavelength database storing, on a per-wavelength basis, information about sections through which optical signals contained in the WDM signal pass, and control unit for changing a setting of the optical add-drop unit so that a path of an optical signal of a wavelength passing through a particular one of the sections bypasses the particular section when all wavelengths stored in the wavelength database are not detected in the particular section by the light wavelength detecting unit.