An optical encoder includes: a light source; a first grating plate having a first diffraction grating for diffracting a light beam emitted from the light source; a second grating plate having a second diffraction grating for further diffracting the light beam diffracted by the first diffraction grating; a reflector for reflecting the light beam from the second grating plate so as to allow the light beam to reenter the second grating plate; and a light-receiving portion for receiving the light beam reflected by the reflector and successively diffracted by the second and first grating plates, wherein a diffraction angle of plus and minus first-order diffracted light beams of the first diffraction grating is substantially equal to that of the plus and minus first-order diffracted light beams of the second diffraction grating, and the light-receiving portion generates an electric signal in accordance with the amount of the plus and minus first-order diffracted light beams of the first diffraction grating.