To increase the power of a signal light without enhancing the output of laser in a laser interferometer to improve S/N ratio.

This laser interferometer comprises a laser beam source 2 for emitting laser beam 3 toward a measuring object 1; a Fabry-Perot interferometer 13 for reflecting and reciprocating a signal light 4 obtained by reflecting the laser beam 3 by the measuring object 1, and signal processing means 16 and 17 for converting the signal light 4 transmitted by the Fabry-Perot interferometer 13 followed by processing. A photo refractive crystal 8 is arranged on the incident path of the signal light 4 to the Fabry-Perot interferometer 13, so that a reference light 9 is crossed to the signal light 4 incident on the photo refractive crystal 8 at a prescribed angle.