Abstract: In a wavelength division multiplexing transmission system, a controller of a relay node of a post-stage notifies level decrease information to a transmission node, upon detecting a decrease in a level of a wavelength in a wavelength multiplex signal received from the transmission node of a pre-stage by the optical channel monitor of the transmission node. The controller of the transmission node of the pre-stage determines that a failure occurs in the wavelength of the transponder of the transmission node, upon detecting a decrease in the level of the wavelength in the wavelength multiplex signal transmitted to the relay node by the optical channel monitor of the transmission node and upon receiving the level decrease information for the wavelength from the relay node. Therefore, a wavelength channel failure in the transponder can be determined in a preferable manner.