A reflectomerry method and device are disclosed for measuring the loss distribution of an optical waveguide on the basis of Rayleigh back-scattered signals from an optical waveguide under test. Light output from a tunable low-coherent. light source, in which laser oscillation is suppressed, is divided into first and second lights. Local oscillator light is generated by propagating the first light over a variable optical path length determined by a movable mirror. The second light is used as a probe light which is launched into the optical waveguide under test. The local oscillator light is combined with light reflected from the waveguide under test. An average value of the Rayleigh back-scattered signals from the waveguide under test is obtained at respective center wavelengths of the light source by measuring the intensity of the combined light while maintaining the variable optical path length constant and varying the center wavelength of the light source. The average value of the Rayleigh back-scattered signals is thus obtained at a point of the waveguide under test. After changing the variable optical path length by incrementally shifting the movable mirror, the above procedure is repeated to obtain an average value of the Rayleigh back-scattered signals at successive points of the waveguide under test. By thus reducing the number of times the mirror needs to be shifted, the time required for measuring the entire waveguide is reduced.