In a one-fiber bidirectional optical transmission system in which output optical signals of optical transmitter-receivers respectively connected to the opposite ends of one optical fiber transmission line are bidirectionally transmitted in the optical fiber transmission line, which utilizes a Raman amplification effect by backward pumping, a frequency satisfying the conditions of |fs1−f0|≠|fp2−f0| and |fs2−f0|≠|fp1−f0| is selected, where f0 is a zero dispersion frequency of the optical fiber transmission line, fs1 and fs2 are the frequencies of the first signal and the second signal, respectively, and fp1 and fp2 are frequencies of the first Raman pump light and the second Raman pump light, respectively.