An optical spectrum analyzer includes a surface elastic wave generator for generating in an optical waveguide a surface elastic wave which has a continuously varying frequency and which diffracts and deflects a guided wave as it travels across the path of the guided wave, which guided wave is introduced as a measured light beam into and travels in the optical wave guide. The measured light beam which has been deflected is emitted from the optical waveguide and detected by a light detector. A modulator turns on and off or modulates the surface elastic wave by repeatedly energizing and de-energizing the surface elastic wave generator, the frequency of the modulation being lower than the continuously varying frequency. A filter receives a signal corresponding to the modulation frequency from the modulator and a signal corresponding to the measured light beam from the detector, and extracts a signal component having the same frequency as the modulation frequency from the signal received from the light detector. The frequency of the surface elastic wave is detected on the basis of the signal component extracted by the filler when the measured light beam is detected by the light detector.