PURPOSE: To decrease the measurement error arising from the fluctuation of a deflection position by providing a detection means for detecting the amplitude of an excited elastic wave and control means for controlling the elastic wave excitation means in the direction to approximate the detection value to a prescribed value and maintaining the specified amplitude of the elastic wave under various fluctuation factors.

CONSTITUTION: A transducer 12 excites the elastic wave propagating in a dielectric 11 by converting the electric signal of an ultrasonic band supplied from a driving power source 13 to the mechanical oscillation of the same frequency. Incident light 5 is Bragg-difrracted by the elastic wave propagating in the dielectric 11 and is made to exit light 6. A transducer 14 converts the elastic wave propagating in the dielectric 11 to the electric signal corresponding to the amplitude thereof and supplies the same to a control circuit 15. The circuit 15 controls the output of the power source 13 in the direction where the amplitude of the electric signal received from the transducer 14 is maintained at the specified value. As a result, the amplitude of the elastic wave propagating in the dielectric 11 is maintained constant and the deflection position of the incident light 5 is also maintained constant. The fluctuation of the deflection position arising from the fluctuation of the amplitude of the ultrasonic wave is thus effectively prevented.