An optical displacement-measuring instrument of a reflection type includes a sensor head movable above a scale in a direction of measurement. This instrument is of a three-grating type that comprises a first optical grating arranged close to a light source, a second optical grating arranged on the scale, and a third optical grating arranged in a photoreceptor. A light diffuser is arranged between the light source and the first optical grating. A collimated light from the light source is diffused at the light diffuser and led to the first optical grating.