In the method and a device for detecting the phase of a moving object using a heterodyne interferometer, a heterogeneous mode helium-neon laser is used as a direct light source to increase a measuring speed, a measuring resolution, and minimize the loss of the light source. Signals, which have only a frequency difference between reference signals or measured signals and arbitrary signals, are extracted from signals which are obtained by multiplying the arbitrary frequency signals by the reference or measured signals. After frequencies of the reference and measured signals are converted, a phase difference of the extracted signals and displacement of the moving object is measured. The system includes a laser light source, an optical interferometer, a frequency converter, and a phase measurer. The light source uses output light, emitted from the laser generator, which is stabilized in frequency, and has two frequencies which are at right angles to each other and linearly polarized.