PURPOSE: To enable the substitution of to the measurement with high accuracy of an audio transmission quantity by the heterodyne detection of two laser beams which are displaced by the frequency F and frequency F+f.

CONSTITUTION: The laser beam emitted from a laser 1 is divided into laser beams 3, 4 by a divider 2. The beam 3 is dispersed by the acoustooptic modulator 5 which is driven by the output of the oscillator 7 having frequency F and generates a dispersing beam 9 and enters into a coupler 11 through the instrument to be measured. On the other hand the beam 4 is dispersed by the acoustooptic modulator 6 which is driven by the output of the oscillator 8 having the frequency F+f and a dispersing beam 10 is caused and enters into the coupler 11. The audio signal 13 having the differential frequency f is then generated by the detector 12. When the beam 10 is found sufficiently big compared to the output beam 17 of the instrument 16 to be measured, the attenuation and phase of the beam 17 are held in the signal 13 and measured by an audio transmission measurer 14. The reference signal 18 for measurement is obtd. with detecting the outputs of the oscillators 7, 8 by a detector 15.