To perform mode hop detection accurately.

Light emitted from a wavelength sweeping light source 20 is received by a light interference section 31, split into two and propagated through optical paths having a predetermined difference of length, before being multiplexed and entered into a light receiver 32. Spectrum analysis of a beat signal appearing in the output from the light receiver 32, due to the difference of optical path length given in the light interference section 31, is performed in a spectrum analysis section 35, and a mode hop determination section 36 determines whether or not there is a mode hop based on the floor noise level of the beat signal obtained by spectrum analysis.