PURPOSE: To measure the loss properties or the like at an optional wavelength of a wide range of wavelengths at low cost and easily by a method wherein the pulse light of large intensity is brought into one end of an optical fiber thereby to cause stimulated Raman scattering, and the scattering light projecting from the other end is used for one end of an optical fiber to be measured.

CONSTITUTION: The pulse light from a fiber Raman laser device 1 is brought into an optical fiber 9 to be measured through an optical attenuator 2. The Rayleigh back scattering light generated at the fiber 9 is separated by a photocoupler or an optical switch 3 and guided to a spectral device 4. Lights of two wavelengths are taken out by the device 4 and sent to a digital averaging circuit 7 via a photodetecting device 5 and an amplifier 6. The circuit 7 performs sampling and A/D conversion of the input signals. Moreover, the circuit 7 has obtained the data of the intensity of the back scattered light since the pulse light was incident upon the fiber 9. The data is sent to a computer 8, and the loss properties at each position in the longitudinal direction of the fiber 9 are displayed.