in fig1 there is shown a waveform of the response light returned to the otdr measuring apparatus 1 this waveform is obtained by recording in time series the response light returned thereto via the coupler 2 in fig1 the waveform is expressed along the horizontal axis obtained by multiplying the time axis by the transfer rate of the light i e the length of the optical fiber along which the response light was propagated here the response light is a mixed light of reflected light from each filter 41 u02dc 48 and these filters are arranged at positions where the distances from the otdr measuring apparatus 1 is different hence the reflected lights from each filter 41 u02dc 48 observed by the otdr measuring apparatus 1 are not overlapped on each other on the time axis and each reflected light is observed separately in fig1 there are shown the waveforms of reflected lights returned to the otdr measuring apparatus 1 via the optical fibers no 1 u02dc no 8 in order from a waveform r shown farthest to the left which is the reflected light from the star coupler toward the right fig1 a and b show in an enlarged scale waveforms of each reflected light returned via the optical fibers no 6 u02dc no 8 of the reflected lights observed by the otdr measuring apparatus 1 fig1 a shows a case where there is no fault in any optical fiber and fig1 b show a case where a fault is simulated by adding a bending loss of 3 db to the optical fiber no 7 as shown in these figures.