In a case where a wavelength to be assigned to a subscriber-side device, to which a downstream wavelength has been assigned, is to be changed from the currently used (Source) downstream wavelength to a different changeover target (Target) downstream wavelength, a downstream wavelength changeover instruction message that indicates the change target wavelength is generated. In a case where a wavelength to be assigned to the subscriber-side device, to which an upstream wavelength has been assigned, is to be changed from the currently used (Source) upstream wavelength to a different changeover target (Target) upstream wavelength, an upstream wavelength changeover instruction message that indicates the change target wavelength is generated. The downstream wavelength changeover instruction message and the upstream wavelength changeover instruction message are respectively independently generated, and only the wavelength for which the wavelength changeover instruction message was generated is changed.