Abstract: The output light from a light source 10 is incident upon an optical fiber to be measured via an optical directional coupler 11, an optical switch 12, an optical amplifier 13, and an optical directional coupler 14. Light from the optical directional coupler 11 is incident upon a polarization controller 16. The output light from the polarization controller 16 and light returned from the optical fiber 15 are incident into an optical balance circuit 17, and these light are interfered. A fixed frequency, or alternatively a frequency which is varied stepwise, is output from a voltage control oscillator 18, according to a control signal from a DC voltage generation circuit 19 or from a saw tooth wave generation circuit 20. The output of the optical balance circuit 17 and the output of the voltage control oscillator 18 are mixed together by a mixer 26.