A spatial frequency reproducing apparatus, comprising: a light source being configured to irradiate an object under measurement with light; a scanning device for two-dimensionally scanning the light from the light source; an optical element (36, 38) located on a tilted optical axis having a tilt with respect to an irradiation optical axis which is an optical axis of a luminous flux of a zero order diffracted light emitted from the object under measurement, the tilted optical axis being located at a middle between the zero order diffracted light and a first order diffracted light, the optical element (36, 38) being configured to make two luminous fluxes formed of the zero order diffracted light and the first order diffracted light interfere with each other; a plurality of photo detectors (40, 41, 50, 51) each detecting the luminous fluxes made to interfere by the optical element (36, 38); characterized in that the spatial frequency reproducing apparatus further comprises: a comparator being configured to: amplify electrical signals of an arbitrary light receiving output photoelectrically converted by the plurality of photo detectors (40, 41, 50, 51) while varying a degree of amplification according to frequencies of the electrical signals, and generate an output summation or an output difference between outputs of the amplified electrical signals of the plurality of photo detectors (40, 41, 50, 51); and a data processing unit being configured to determine each of the output summation and the output difference to obtain a measurement value. a light source being configured to irradiate an object under measurement with light; a scanning device for two-dimensionally scanning the light from the light source; an optical element (36, 38) located on a tilted optical axis having a tilt with respect to an irradiation optical axis which is an optical axis of a luminous flux of a zero order diffracted light emitted from the object under measurement, the tilted optical axis being located at a middle between the zero order diffracted light and a first order diffracted light, the optical element (36, 38) being configured to make two luminous fluxes formed of the zero order diffracted light and the first order diffracted light interfere with each other; a plurality of photo detectors (40, 41, 50, 51) each detecting the luminous fluxes made to interfere by the optical element (36, 38); characterized in that the spatial frequency reproducing apparatus further comprises: a comparator being configured to: amplify electrical signals of an arbitrary light receiving output photoelectrically converted by the plurality of photo detectors (40, 41, 50, 51) while varying a degree of amplification according to frequencies of the electrical signals, and generate an output summation or an output difference between outputs of the amplified electrical signals of the plurality of photo detectors (40, 41, 50, 51); and a data processing unit being configured to determine each of the output summation and the output difference to obtain a measurement value. a comparator being configured to: amplify electrical signals of an arbitrary light receiving output photoelectrically converted by the plurality of photo detectors (40, 41, 50, 51) while varying a degree of amplification according to frequencies of the electrical signals, and generate an output summation or an output difference between outputs of the amplified electrical signals of the plurality of photo detectors (40, 41, 50, 51); and amplify electrical signals of an arbitrary light receiving output photoelectrically converted by the plurality of photo detectors (40, 41, 50, 51) while varying a degree of amplification according to frequencies of the electrical signals, and generate an output summation or an output difference between outputs of the amplified electrical signals of the plurality of photo detectors (40, 41, 50, 51); and a data processing unit being configured to determine each of the output summation and the output difference to obtain a measurement value.