PURPOSE: To suppress the generation of back-talk by forming an optical frequency shifter (ultrasonic modulating element) for polarizing the frequency of laser light to the prescribed size in a laser light path formed between a laser light source and a measured substance upon which the laser light is made incident approximately vertically through an optical system.

CONSTITUTION: The frequency of primary diffracted light D1 corresponding to the laser light incident on the ultrasonic modulating element 2 is set up to the size obtained by shifting the frequency of the incident laser light by the frequency of an ultrasonic signal. When the primary diffracted light D1 delivered from the element 2 is reflected by a moving body 9, returned and then made incident on the element 2, its frequency is shifted and the frequency of the incident laser light is different from the frequency fixed by a resonator in a light source 1, so that back-talk is not generated.