In a measurement requiring a high space resolution using S-BOTDR, a pulse train composed of a plurality of pulses having the interval between the pulses longer than the phonon lifetime is interpulse-code-modulated. A Golay code is used for the interpulse code modulation to eliminate the sidelobes of the correlation in using a technique of correlation. In a technique without using correlation, an Hadamard matrix is used for the interpulse code modulation and the resultant matrix is inverted in the signal processing.