A light image of a scale, which bears length or angle information recorded in the form of a bright and dark bit pattern, is projected onto the light receiving surface of a storage effect type sensor, whereby a charge pattern data corresponding to the projected image is formed. Before the regular read-out of the charge pattern data thus formed from the sensor, the sensor is scanned at least once at a higher scanning frequency than the frequency for the regular data read-out. From the data thus read out, read-out data that obtained in the absence of illumination light is subtracted, and the resultant data which is free from error component is processed to obtain a final measurement of the length or angle.