PURPOSE: To facilitate adjustment by providing a fixedly arranged couple of slits, a couple of polarizers which have polarization directions crossing with each other at right angles, and a mechanism which generates a couple of light beams alternately at a specific periods as a modulating means of luminous flux incident to a photoelectric transducer.

CONSTITUTION: Luminous flux from a scale line 42 on a scale 41 is incident to the photoelectric transducer 58 through slits 54a and 54b of a slit plate 54 alternately by the irradiation with alternate polarized light beams P' and Q' from a light irradiating mechanism which are polarized linearly at right angles to each other and the operation of the slits 54a and 54b of the slit plate 54 and polarizing plates 56a and 56b. This when viewed from the side of the photoelectric transducer 58 is equivalent to the effect when one slit is oscillated mechanically corresponding to a detection origin and the displacement of the scale line 42 on the scale is therefore detected.