PURPOSE: To provide a method for measuring the amount of optical displacement that is hard to be influenced by a disturbance light.

CONSTITUTION: A light is emitted on a scale 90 that is provided with parts having different two optical characteristics which are periodically arranged one after the other so that a periodic image pattern is formed thereon. The periodic image pattern is detected by a one-dimensional image sensor 94, then the amount and the direction of relative displacement between the scale 90 and the image sensor 94 are judged from the variation of the periodic image pattern that is detected by the image sensor 94. The magnitudes of outputs of adjacent pixels of the image sensor are compared with each other so that a measurement pulse corresponding to the periodic image pattern is formed at a time point when the relationship of the magnitudes is inverted. The amount and direction of the relative displacement between the scale 90 and image sensor 94 are judged from the amount and the direction of movement of each unit of pixel of the image sensor 94 of the measurement pulse.