A displacement detecting device has a diffraction grating, grating interferometers, and relative position information output sections. The grating interferometers have a light source, reflectors, a beam splitter, and light receiving sections. The reflectors reflect 1st-order diffracted lights diffracted by the diffraction grating, and cause the reflected 1st-order diffracted lights to be incident at a position substantially identical to the position at which the light from the light source is irradiated. Furthermore, the reflectors cause the 1st-order diffracted lights to be incident on the diffraction grating at an angle different to either the incidence angle of the light from the light source incident on the diffraction grating or the angle at which the 1st-order diffracted lights are transmitted through or reflected by the diffraction grating.