PURPOSE: To measure speeds of a plurality of objects to be measured only by using one light modulator, by applying photoelectric conversion to a synthetic light of a plurality of reference light beams obtained by splitting a reference light beam from one light modulator, and of a plurality of measured light beams.

CONSTITUTION: A light beam LB2 has a component EP and a polarized component ES and is modulated by a light modulator 35, and a light beam LB3 is separated into a light beam LB4 and a light beam LB5 which has only the component ES in a polarization beam splitter 36. The beam LB4 is made to enter one end of a polarized plane retaining fiber 38A and applied to a measuring point P1 of a speaker 1, and a light reflected therefrom returns to the beam splitter 36 as a measured light beam LB4*. The beam LB5 is applied likewise to a measuring point P2 of the speaker 1, and a light reflected therefrom returns to the beam splitter 36 as a measured light beam LB5*. These beams LB4* and LB5* are reflected by about 50% by a beam splitter 34, the beam LB4* is transmitted as it is, while the beam LB5* is totally reflected, and they are made to enter light-detecting elements 19A and 19B respectively. The elements 19A and 19B subject the synthetic lights to photoelectric conversion and supply them to measurement signal forming circuits 21A and 21B.