PURPOSE: To make an optical transmission level stable even when a signal component of a wavelength is decreased.

CONSTITUTION: The system consists of plural optical transmission panels 1, 2 applying amplitude modulation of sine waves whose frequencies are low and different to digital optical signals with different wavelengths, an optical synthesizer 3 synthesizing output signals of the transmission panels 1, 2, an optical fiber amplifier 4 amplifying the output signal of the optical synthesizer 3 in the lump, an optical branching device 5 branching part of the optical signal outputted to the transmission line by the output of the amplifier 4 as a monitor signal, a photodiode 6 converting the branched optical signal into an electric signal, band pass filters 8, 9 extracting the sine wave with the low frequency subject to amplitude modulation, peak value detection circuits 10, 11 converting the output sine wave of the filters 8, 9 into a DC voltage in response to the amplitude and operational amplifiers 12, 13 as a negative feedback circuit comparing an output current voltage of the circuits 10, 11 with a reference voltage and controlling the optical transmission panels 1, 2 so as to minimize the voltage difference.