A photoelectric encoder comprising: an irradiation unit configured to apply first and second irradiation light beams (L1a, L1b) having a first linear polarization direction; a scale configured to produce first and second diffraction light beams (L2a, L2b) having the first linear polarization direction by diffracting the first and second irradiation light beams (L1a, L1b), respectively; a polarizing unit configured to convert the first diffraction light beam (L2a) into a third diffraction light beam (L2c) having a second linear polarization direction which is perpendicular to the first linear polarization direction, to produce first and second composite light beams (L3a, L3b) by combining the second diffraction light beam (L2b) and the third diffraction light beam (L2c), and to convert the first composite light beam (L3a) into a circularly polarized third composite light beam (L3c); and a light receiver configured to receive the second composite light beam (L3b) and the third composite light beam (L3c); characterised in that the scale has a glass plate whose front surface has a grating shape formed by projections and recesses and in that the polarizing unit comprises: a third mirror configured to reflect the first diffraction light beam (L2a); a fourth mirror configured to reflect the second diffraction light beam (L2b); and a half-wave plate configured to convert the first diffraction light beam (L2a) reflected from the third mirror into the third diffraction light beam (L2c) having the second linear polarization direction by rotating a polarization direction of the first diffraction light beam (L2a). an irradiation unit configured to apply first and second irradiation light beams (L1a, L1b) having a first linear polarization direction; a scale configured to produce first and second diffraction light beams (L2a, L2b) having the first linear polarization direction by diffracting the first and second irradiation light beams (L1a, L1b), respectively; a polarizing unit configured to convert the first diffraction light beam (L2a) into a third diffraction light beam (L2c) having a second linear polarization direction which is perpendicular to the first linear polarization direction, to produce first and second composite light beams (L3a, L3b) by combining the second diffraction light beam (L2b) and the third diffraction light beam (L2c), and to convert the first composite light beam (L3a) into a circularly polarized third composite light beam (L3c); and a light receiver configured to receive the second composite light beam (L3b) and the third composite light beam (L3c); characterised in that the scale has a glass plate whose front surface has a grating shape formed by projections and recesses and in that the polarizing unit comprises: a third mirror configured to reflect the first diffraction light beam (L2a); a fourth mirror configured to reflect the second diffraction light beam (L2b); and a half-wave plate configured to convert the first diffraction light beam (L2a) reflected from the third mirror into the third diffraction light beam (L2c) having the second linear polarization direction by rotating a polarization direction of the first diffraction light beam (L2a). a third mirror configured to reflect the first diffraction light beam (L2a); a fourth mirror configured to reflect the second diffraction light beam (L2b); and a half-wave plate configured to convert the first diffraction light beam (L2a) reflected from the third mirror into the third diffraction light beam (L2c) having the second linear polarization direction by rotating a polarization direction of the first diffraction light beam (L2a).