An optical encoder includes a detection head. The detection head includes an optical system including a lens including a pair of bosses integrally formed with the lens and disposed with an optical axis of the lens therebetween, and an aperture plate having a pair of reference-pin-insertion holes disposed with an aperture therebetween; a housing including a pair of attachment portions and having a pair of lens-boss-insertion holes and a pair of reference-pin-insertion holes disposed on a central symmetry plane; a lens holder having a pair of lens-boss-insertion holes and a pair of reference-pin-insertion holes disposed on the central symmetry plane; and a pair of reference pins for positioning the lens holder and the aperture plate relative to the housing. An optical axis of the optical system is adjusted with respect to the bosses and the reference pins.