PURPOSE: To make it possible to measure very long fiber, by generating a sampling signal based on a clock signal, fixing the sampling signal, and moving an optical pulse triggering signal at every sampling by a minute time.

CONSTITUTION: In this meter, an optical pulse generator 15 is operated by the output of an analog delay circuit, i.e., a second delay circuit 14. Meanwhile, a sampler 18 is operated by the output of a digital delay circuit, i.e., a first delay circuit 13. Therefore, the sampler 18 and an O/E input amplifier 17 do not receive adverse effect of the operation of the analog delay circuit 14. Even if any signal, which is synchronized with the clock pulses, is inputted to the O/E input amplifier 17 and the sampler 18, the signal only changes the offset of the output of the sampler 18 by a specified value, and errors are not yielded. Thus even extremely weak back scattered signal can be accurately sampled, and very long fiber can be measured.