PURPOSE: To perform highly accurate displacement detection, by arranging light sources and first and second slant stripes, which are slanted in the reverse direction to each other between image sensors in a V shape, and adding the amounts of movements of light beams, which are transmitted through the stripes.

CONSTITUTION: On a movable body 10, condenser lenses 18r and 18l, which collects the reflected light beams from a reference scale 13, and first and second image sensors 19r and 19l are attached. Slant stripes 16r and 16l are provided between light sources 11r and 11l and image sensors 19r and 19l in a V shape. The light beams, which are transmitted through the slant stripes 16r and 16l are detected by the image sensors 19r and 19l, and the amounts of movements are added. In this way, the relative movement of the movable body 10 with respect to the reference scale 13 is detected.