PURPOSE: To remove causes of errors, by guiding light beams, which are propagated in the reverse directions in an optical fiber loop to the same optical axis, thereafter inputting the optical beams into an acoustooptic element, and detecting the phase difference between the specific detected signals with regard to the driving frequency.

CONSTITUTION: Light from a light source 1 is split into two directions by a beam splitter 3, and the light beams are propagated in the reverse directions in an optical fiber loop 6. The light beams are made to pass through the splitter 3 again and inputted into an acoustooptic element 8 that is driven by an ultrasonic driving signal 13. The light beams undergo Bragg diffraction and are circularly polarized by a 1/4 wavelength plate. Then the beams are detected by a detector 11 through an analyzer 10. Then, the phase difference between the signal, which is obtained by dividing a detected signal 12 from the detector 11 into 1/2N (where N is a natural number), and the signal, which is obtained by dividing a driving signal 13 from the element 8 into 1/N, is detected by a phase comparator 17. In this way, the effects of fluctuation of the amount of light of the light source and the like can be removed.