To provide an optical encoder capable of obtaining desired precision by regulating the relation among the necessary phase precision, distance between index scale and main scale, divergent angle of a parallel light beam, and a period of a periodic pattern.

The optical encoder is characterized in that the value expressed by the phase of photoelectric output signal as the ratio of the divergent distance value to the pitch P of the main scale 42 (g×tanθ)/P, where the divergent distance (g×tanθ) of the parallel light beam with beam divergent angle θ which is emitted from the collimator 41 while passing through the index scale 43 till arriving at the main scale 42 across a gap g, is smaller than the necessary phase precision Aph of the optical encoder 45.