PURPOSE: To lower an optical output required to an optical amplifier, to avoid a limit in a division number due to gain saturation and to obtain an output having a sufficient S/N by providing demultiplexers bisecting a signal light and optical amplifiers alternately.

CONSTITUTION: Demultiplexers 6a-6c bisecting an optical signal are employed and optical amplifiers 7a-7f amplifying each of bisected optical signals are provided on the transmission network and the demultiplexers 6a-6c and the optical amplifiers 7a-7f are provided alternately. In this case, the gain of the rare-earth dope fibers 7a-7f with respect to a signal light is adjusted to compensate the distribution loss at the X demultiplexers 6a-6c. Since the optical output required to the optical amplifiers 7a-7f is low, the limit in the division number due to gain saturation is not generated. Since the average photon number is not largely decreased, the deterioration in the S/N of the output due to a quantization noise limit is not generated, as well.