Laser light output from a laser light source is irradiated onto a transmission-type
scale. Due to movement of the transmission-type scale, a polarization angle of
transmitted light varies in accordance with variations in orientations of half-wave
plates arrayed in a longitudinal direction. It is difficult for the polarization
angle to be affected by noise factors such as external light or the like. Light
intensity of a polarized light component, which is transmitted through an analyzer
and detected at a photo-detector, varies in accordance with the polarization angle.
Thus, the light intensity varies as the transmission-type scale moves, and a detection
signal is output to a movement amount computing device. When an amount of movement
is computed on the basis of the signal, the exact amount of movement can be detected.
