Abstract: An optical encoder includes a to-be-detected medium including slits to reflect and transmit light. An optical source radiates light to the to-be-detected medium. A photodetector detects the light reflected by the slits or transmitted through the slits, and generates an electrical signal corresponding to an amount of the detected light. The correction value storage stores an offset correction value used to eliminate or reduce an offset component contained in the electrical signal. The adjustor adjusts an amount of the radiated light or the offset correction value so that a ratio between the offset correction value and the amount of the radiated light is approximately constant. The signal corrector corrects the electrical signal corresponding to the amount of the adjusted light, or corrects the electrical signal using the adjusted offset correction value. The position detector detects a position of the to-be-detected medium.