In an optical transmission system which can keep a flat pass characteristic within an optical signal modulation band, a spectrum of a dropped optical signal within a modulation band per channel is monitored, and non-flatness for the optical signal is compensated by detecting the non-flatness of a pass characteristic of a transmission line from the spectrum. Another optical fiber has a chromatic dispersion higher than a positive value x and a negative chromatic dispersion slope at a given wavelength band. Bosses joined to the flange and rib are compressed against the bead to bias the rib into tight sealing relation with the bead.