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 |f_s1-f₀||f_p2-f₀| and |f_s2-f₀||f_p1-f₀| is selected, where f₀ is a zero dispersion frequency of the optical fiber transmission line, f_s1 and f_s2 are the frequencies of the first signal and the second signal, respectively, and f_p1 and f_p2 are frequencies of the first Raman pump light and the second Raman pump light, respectively.