<p>An encoder comprises interlockingly rotating first and second rotators each having a slit, a light source emitting light to be detected to the slit, and a photodetector having first and second scales where a plurality of light detecting elements are arranged along an annular arrangement line and a section delivering an output signal based on the intensity of the light to be detected passing through the slit and entering the light detecting elements of the first and second scales. Rotary ratio of the second rotor to the first rotor is different, and the light detecting element is attribute-allocated for every predetermined phase angle.</p>
