by 90 degrees reciprocates twice between the interferometer and a measuring reflector 2 specifically the light beam f 2 led to the reflector 2 on the opposite side by three beam bender 4 5 6 is reflected by the reflector 2 subsequently returns along the same optical path and passes through a quarter wavelength plate 14 twice so that the returning light goes via the polarization beam splitter 3 to the corner cube prism 17 and then returns along the same path and again impinges on the measuring reflector 2 is reflected by the measuring reflector 2 to return again to the polarization beam splitter 3 since the resulting light has the plane of polarization rotated further by 90 degrees the light is bent by the polarization beam splitter 3 at this time and returns to the laser light source ls a portion of the returning light is separated by the non polarization beam splitter 16 and incident on the photodetector 10 this configuration provides the resolution four times as high as a single beam interferometer in a basic configuration fig7 illustrates a further exemplary modification for measuring run out of the axis of rotation to which the differential single beam two pass interferometer is applied members indicated by the same reference numerals in fig7 and 6 are identical in this exemplary modification a measuring reflector is applied to columnar reflection planes 1 a 2 a instead a measuring reflector is applied to columnar reflection planes 1 a 2 a instead of an object b as a cylindrical rotor rb the reflection planes 1 a 2 a are side surfaces arranged back to back to each other of the rotor the diameter of which is positioned on a measuring axis the measuring axis is oriented to.