A power-down delay unit for delaying cutoff of a voltage is provided in a power supply line which supplies electrical power to an optical module of a transponder unit for each optical wavelength. The power-down delay unit assigns different delays for each wavelength. The length of the delay is a time required by an AMP unit to change from an ALC mode to an AGC mode or longer. When the power sources for supplying electrical power to the transponder units becomes faulty and outputs of several wavelengths are simultaneously going to stop, the power-down delay unit delays cutoff of the supply voltage to the optical modules by keeping supply voltage for a while to the optical modules on behalf of power sources enabling the optical modules to operate after the power sources shutdown, so that the actual optical signals gradually stop one by one.