A method for measuring displacement, comprising: turning light emitted from a light source into parallel light by a collimator lens; dividing the parallel light by a first transmissive diffraction grating disposed on an optical axis of the parallel light into zeroth-order light, which travels in the same direction as that of said parallel light, and ±nth-order light having diffraction angles relative to said zeroth-order light, where n is a positive integer of 1 or greater; dividing, by a second transmissive diffraction grating (18; 102; 110), the zeroth-order light and the ±nth-order light that have passed through the first transmissive diffraction grating into zeroth-order light and ±nth-order light, the second transmissive diffraction grating (18; 102; 110) having a same grating pitch as the first transmissive diffraction grating and being disposed so as to face and be movable relative to said first transmissive diffraction grating; characterized by receiving, by a first optical sensor, among diffracted light rays (22, 26; 223, 225) that have passed through both the first and second transmissive diffraction gratings (16, 18; 102; 110), at least one pair of interfering light rays (30; 228) travelling along an optical axis of one of the zeroth-order light, the positive nth-order light, and the negative nth-order light from the first transmissive diffraction grating to detect an amount of light received, said one pair of interfering light rays (30; 228) having mutually overlapping portions when emerging from the second transmissive diffraction grating (18; 102; 110), optical path lengths of said one pair of interfering light rays (30; 228) reaching the first optical sensor being different from each other, and a difference in the optical path lengths being dependent on a displacement between the first and second transmissive diffraction gratings (16, 18; 102; 110) in the direction of the optical axis of the parallel light so that an amount of interference occurs between said one pair of interfering light rays (30; 228) in said mutually overlapping portions depending on the displacement between the first and second transmissive diffraction gratings (16, 18; 102; 110), wherein said first optical sensor is disposed at a location where said mutually overlapping portions of the one pair of interference light rays (30; 228) from the second transmissive diffraction grating (18; 102; 110) are present; and determining an amount of the displacement between the first and second transmissive diffraction gratings (16, 18; 102; 110) in the direction of the optical axis of the parallel light on a basis of said amount of light received by the first optical sensor. turning light emitted from a light source into parallel light by a collimator lens; dividing the parallel light by a first transmissive diffraction grating disposed on an optical axis of the parallel light into zeroth-order light, which travels in the same direction as that of said parallel light, and ±nth-order light having diffraction angles relative to said zeroth-order light, where n is a positive integer of 1 or greater; dividing, by a second transmissive diffraction grating (18; 102; 110), the zeroth-order light and the ±nth-order light that have passed through the first transmissive diffraction grating into zeroth-order light and ±nth-order light, the second transmissive diffraction grating (18; 102; 110) having a same grating pitch as the first transmissive diffraction grating and being disposed so as to face and be movable relative to said first transmissive diffraction grating;