PURPOSE: To measure the characteristics of optical fiber at a specific wavelength and easily change the oscillation wavelength of a semiconductor laser.

CONSTITUTION: As for the light source 1 for an OTDR, an optical fiber laser 5, which has a resonator 4 provided with optical fiber 2 and a wavelength filter 3 which allows variable transmission wavelength, is used. Since the spectrum of a narrow-width of 1nm or narrower can be easily provided, the characteristics of the optical fiber at a specific wavelength are examined. Since the wavelength of the output light from a light source 1 can be changed, the characteristics of the optical fiber at the desired wavelength are examined. A polarized wave modulator 7 is inserted without cutting the optical fiber and the constitution of the optical fiber laser 5 is simplified.