A displacement detecting device comprising: a scale diffraction grating; and a detecting head unit that is disposed so as to be relatively movable with respect to the scale diffraction grating, and detects the amount of relative displacement to the scale diffraction grating, wherein the detecting head unit includes a light source that emits light toward the scale diffraction grating, a first retroreflecting unit that retroreflectes positive s-order diffracted light of light diffracted by the scale diffraction grating, such that the retroreflected light enters the scale diffraction grating again, a second retroreflecting unit that retroreflectes negative s-order diffracted light of the light diffracted by the scale diffraction grating, such that the retroreflected light enters the scale diffraction grating again, wherein "s" is an integer of 1 or greater, and a light receiving unit that receives an interference fringe formed on the light receiving unit by the light retroreflected by the first retroreflecting unit and diffracted by the scale diffraction grating and the light retroreflected by the second retroreflecting unit and diffracted by the scale diffraction grating, and each of the first retroreflecting unit and the second retroreflecting unit has a deflecting function of deflecting light incident on the corresponding retroreflecting unit (350, 360) by a predetermined angle and then emitting the light. a scale diffraction grating; and a detecting head unit that is disposed so as to be relatively movable with respect to the scale diffraction grating, and detects the amount of relative displacement to the scale diffraction grating, wherein the detecting head unit includes a light source that emits light toward the scale diffraction grating, a first retroreflecting unit that retroreflectes positive s-order diffracted light of light diffracted by the scale diffraction grating, such that the retroreflected light enters the scale diffraction grating again, a second retroreflecting unit that retroreflectes negative s-order diffracted light of the light diffracted by the scale diffraction grating, such that the retroreflected light enters the scale diffraction grating again, wherein "s" is an integer of 1 or greater, and a light receiving unit that receives an interference fringe formed on the light receiving unit by the light retroreflected by the first retroreflecting unit and diffracted by the scale diffraction grating and the light retroreflected by the second retroreflecting unit and diffracted by the scale diffraction grating, and a light source that emits light toward the scale diffraction grating, a first retroreflecting unit that retroreflectes positive s-order diffracted light of light diffracted by the scale diffraction grating, such that the retroreflected light enters the scale diffraction grating again, a second retroreflecting unit that retroreflectes negative s-order diffracted light of the light diffracted by the scale diffraction grating, such that the retroreflected light enters the scale diffraction grating again, wherein "s" is an integer of 1 or greater, and a light receiving unit that receives an interference fringe formed on the light receiving unit by the light retroreflected by the first retroreflecting unit and diffracted by the scale diffraction grating and the light retroreflected by the second retroreflecting unit and diffracted by the scale diffraction grating, and each of the first retroreflecting unit and the second retroreflecting unit has a deflecting function of deflecting light incident on the corresponding retroreflecting unit (350, 360) by a predetermined angle and then emitting the light.