Gain setting of a receiving amplifier, is performed by detecting the
necessity of gain setting when a receiving amplifier is turned on,
requesting WDM transmission equipment in a preceding station to output
ASE light; in a WDM transmission equipment of the preceding station,
shutting off both passing-through light and added light, and outputting
the ASE light corresponding to a predetermined number of wavelengths of
signal light; in the receiving amplifier of the WDM transmission
equipment in the station of interest, performing the gain setting by use
of the ASE light; and on completion of the gain setting, the WDM
transmission equipment of the station of interest, requesting the WDM
transmission eouipment of the preceding station to halt the ASE light
output, and the WDM transmission equipment of the preceding station,
halting the ASE light output upon receiving the request and switching the
output to an optical signal output.