directional loss distribution in both directions of the transmission lines a and b by means of the otdr measurement unit 40 of the a station by transmitting the wavelengths u03bba and u03bbb in the direction a of the transmission line a from the otdr measurement unit 40 backscattering light of the wavelength u03bba transmitted from the b station in the transmission line b is taken from the transmission line b by means of the optical coupler 30 immediately after the optical amplifier 28 and is coupled in the optical transmission line a opposite line immediately before the optical amplifier 22 through an optical filter 36 which passes therethrough only the wavelength u03bba or blocks the wavelength u03bbb in such a manner as to travel in the direction a of the transmission line a by means of the optical coupler 20 the wavelength u03bbb transmitted from the b station to the transmission line b is taken from the transmission line b by means of the optical coupler 26 and is coupled in the optical transmission line a opposite line immediately before the optical amplifier 22 through the optical filter 43 which passes therethrough only the wavelength u03bbb or blocks the wavelength u03bba in such a manner that the wavelength u03bbb travels in the direction b of the transmission line a by means of the optical coupler 20 backscattering light of the wavelength u03bbb coupled in the b direction of the transmission line a travels in the transmission line a in the a direction thereby it is possible to monitor a longitudinal directional loss distribution in both directions of the transmission lines a.