PURPOSE: To simplify operation by measuring an angle and plane angle by using a plane mirror on the side of a body to be measured.

CONSTITUTION: Laser beams of frequencies f1 and f2 differing in deflection surface are supplied from laser beam generator 21 to beam splitter 24 of interference system 22. The beam of frequency f1, having passed through beam splitter 24, is reflected by plane mirror 29 of measured body 23 and, after being Doppler-shifted, supplied to beam splitter 24 again. Further, the incident beam, reflected by beam splitter 24, advances to plane mirror 29 again by way of cube corner 26 and beam splitter 24 and is reflected there and after the Doppler-shift, the light is caused to strike beam splitter 24 before photodetector 30. Similarly, the light of frequency f2 is caused to strike photodetector 30. Photodetector 30 provides the differential Doppler-shift and from its output signal, the angular displacement of measured body 23 is detected.