It is extraordinary useful to conserve the advantages of the heterodyne interferometry with high sensitivity of straight phase measurements in oblique incidence interferometer. In our case the directions of test and referent beams are made by the diffraction gratings. The special-property phase plate is located in collimated beam path between the diffraction gratings and test mirror, and incidence test and referent beams pass through this plate twice. The particularity of the phase plate is that one creates the path difference (lambda) /4 at reference beam direction, and (lambda) /2 at test beam direction. Initial polarization vector of reference beam is rotated on 90 degree(s) and polarization vector of test beam on 180 degree(s). Such two beams with different frequency interfere at the exit. It has been found optimum decision of polarization frequency component dividing and combining.