A device for measuring a moving distance of two relatively moving objects includes a first diffraction grating provided on one of the two objects and disposed along the relatively moving direction of the two objects, and a measuring portion provided on the other object. The measuring portion includes a second diffraction grating, a light source and a photodetecting system, wherein the light source provides lights which are projected upon two points on the second diffraction grating so that they emanate from the two points in the form of diffraction lights having different diffraction orders. The diffraction lights are directed to the same point on the first diffraction grating and are diffracted again by the first diffraction grating so that they are emitted in the same direction, and the photodetecting system is operable to detect a change in the light intensity caused due to the interference of the two lights emanating from the first diffraction grating. The device further includes a detecting system for detecting the relative moving distance of the two objects on the basis of the detection by the photodetecting.