PURPOSE: To eliminate the need for beam split optical-parts such as a Wollaston prism and to remove mixture of polarization leakage component with beat signal, in the case where a differential type heterodyne interferometer is configured.

CONSTITUTION: Relating to the beam having passe a laser light source LS and a 1/2 wavelength board HW, the electric vector is made incident on a polarizaing axis of a polarization beam splitter PBS1 at an angle of 45°, and splitted in two. There surface frequency deviation at acoustic optical elements A01 and A02, respectively. Each beam becomes elliptic polarized light due to imperfectivity of the polarization beam splitter PBS1 and distortion in the acoustic optical element, etc. To remove influence of polarization leakage component at a polarization beam splitter PBS2, the orthogonal two frequency light source, where these outgoing beams are made to cross each other at a constant intersection angle, different from conventional light sources, is provided.