A transponder in a wavelength division multiplexing apparatus detects a wavelength of a leak light (received light) having one of plural wavelengths multiplexed or divided by a WDM unit, the leak light having been received from the WDM unit when the transponder is connected to the WDM unit; determines a transmission wavelength which is a wavelength of a light to be transmitted to the WDM unit on the basis of the wavelength of the received light; and sets to convert the wavelength of the light to be transmitted to the WDM unit into the transmission wavelength, and transmit the converted light.