According to the present invention, there is provided a heterodyne interferometer comprising a laser oscilator, a frequency modulator which splits the laser beam into two beams and causes the beams to have different frequencies, a photodetector which composes both beams after at least one beam is reflected or transmitted by an object under test, and produces an interference signal from a composed laser beam, a frequency analyzer which reveals a Doppler frequency component of the interference signal, and an x-axis and y-axis fine movement command circuit and a photodetector positioning mechanism which detects the amplitude of the a.c.(alternating current) component of the interference signal and positions the photodetector so that the amplitude is maximum. The present invention also provides the heterodyne interferometer which, instead of the x-axis and y-axis fine movement command circuit and a photodetector positioning mechanism, comprises a plurality of photodetector disposed in the direction perpendicular to the laser beam axis, a peak value discriminator which discriminates a peak amplitude of the interference signals output by the photodetectors, and a multiplexer which discriminates interference signal having the largest amplitude and transmits this interference to the frequency analyzer. The heterodyne interferometer is capable of obtaining interference signal of a predetermined level irrespective of the surface condition of an object under test at the laser beam focal point.