A method and apparatus for detecting a frequency change of a light modulates the scattered light from the object of measurement using an acousto-optic modulator or the like, heterodyne-detects the scattered light after modulation to provide an electrical signal having a frequency (f.sub.d -f.sub.m) which is the difference between the Doppler frequency f.sub.d and the modulation frequency f.sub.m, passes this electrical signal to a filter while changing the modulation frequency f.sub.m, and obtains the modulation frequency f.sub.m at which the signal voltage is the largest or the smallest, that is, the Doppler frequency f.sub.d.