a displacement measuring device allows a plurality of light beams to be incident at positions on a scale or diffracted at points on a scale grating , which are farther spaced away from each other than the diameter of the light beams on the scale grating , and makes an angle of incidence of each light beam on the scale equal generally to an angle of transmission of the diffracted light beam of each light beam . this makes the strength of a detection signal impervious to variations in pitch angle and allows for providing good signals . accordingly , it is possible to attach the device easily to an apparatus and provide improved ease of use for the device .