An optical element has three operation modes of light oscillation, light amplification, and light attenuation, which are switched in accordance with the intensity of an excitation component. The optical element having a first light terminal from which a light pulse is output to an optical fiber to be tested, and a second light terminal through which reflected light from the optical fiber is passed in the amplification mode or in the attenuation mode. Light-receiving section receives the reflected light output from the second light terminal of the optical element, and converts the reflected light into an electric signal. Signal processing section subjects the electric signal output from the light-receiving section to a predetermined signal processing step for testing the optical fiber. Excitation component control section outputs to the optical element a first excitation component for enabling the optical element to operate in the oscillation mode at a first timing, a second excitation component for enabling the optical element to operate in the attenuation mode at a second timing subsequent to the first timing, and a third excitation component for enabling the optical element to operate in the amplification mode at a third timing subsequent to the second timing.