PURPOSE: To enable a device to be miniaturized by simplifying a signal- processing circuit system, an unneeded signal which is generated halfway through the signal processing to be eliminated, and sensitivity for detecting speed to be improved.

CONSTITUTION: A laser Doppler speed meter 20 emits light beam which is discharged from a laser light source to an object to be measured and obtains its reflection light, at the same time obtains a reference light by allowing the light beam which is discharged from the laser light source to a phase modulator, and allows a speed of the object to be measured to be measured based on a light-detection signal which is obtained by emitting a synthesis signal of the reflection and reference lights to a light-detection means 8. A modulation signal which is applied to electrodes 4a and 4b of a phase modulator 4 is generated by two sinusoidal wave signals with different frequencies.