A displacement detector device has a source of laser beam; a diffraction grating for separating the laser beam into a plurality of beam components; a filter for shielding the beam components other than the two beam components of .+-. primary order produced by the diffraction grating; a wavelength plate operable to vary the polarized state of one of the two beam components of .+-. primary order; a lens element for focusing the two beam components of .+-. primary order on a displacement plane and a reference plane, respectively; and a photosensor. The two beam components of .+-. primary order focused on the reference and displacement planes, respectively, are subsequently reflected therefrom and are then collimated by the lens element after having again entered the lens element in substantially overlapped relationship. The two beam components emerging from the lens element are again diffracted by the diffraction grating, and the photosensor detects an interfering component of the beam components of primary order.