PURPOSE: To prevent consumption of undesired power by providing a pilot signal generating means and a band pass filter and controlling a gain of a desired optical direct amplifier repeater among plural optical direct amplifier repeaters with a pilot signal passing through the band pass filter.

CONSTITUTION: Both terminal stations 1, 2 are provided respectively with pilot signal generating means 1a, 2a generating plural pilot signals whose frequency different from each other and of the same number as plural optical direct amplifier repeaters 10A-10N. Moreover, band pass filters 16a-16n whose frequency pass band width differs from each other and corresponds to any of frequencies of plural pilot signals are provided to the plural optical direct amplifier repeaters 10A-10N. Then a gain of desired optical direct amplifier repeaters 10A-10N among the plural optical direct amplifier repeaters 10A-10N is controlled by a pilot signal passing through the band pass filters 16a-16n. Thus, the gain of all the optical direct amplifier repeaters 10A-10N is selected to be a desired gain and consumption of undesired power is prevented.