An optical constant measurement apparatus comprising: a coherent light source structured to output coherent light including high-order harmonics obtained by irradiating short-pulse laser light to a nonlinear medium; a spectrometer comprising a grating structured to diffract the coherent light and an image sensor structured to measure an image of light diffracted by the grating; a first double slit having a pair of apertures arranged with an interval in a first direction, which are arranged at a predetermined position between the coherent light source and an incident slit of the spectrometer, in a first state; a second double slit having a pair of apertures that are a replica of the first double slit, which are arranged at the predetermined position as a replacement of the first double slit in a state in which one from among the pair of apertures holds a sample, in a second state; and a calculation processing device structured to calculate optical constants of the sample based on a first interference image formed on the image sensor due to the harmonics of the coherent light in the first state and a second interference image formed on the image sensor due to the harmonics in the second state, wherein the optical constant measurement apparatus is modeled using parameters, wherein the calculation processing device calculates an intensity distribution formed on the image sensor for each of the first state and the second state using a one-dimensional Fresnel diffraction expression based on the model of the optical constant measurement apparatus, and wherein the parameters of the model and the optical constants of the sample are measured such that the intensity distributions thus calculated approach the first interference image and the second interference image. a coherent light source structured to output coherent light including high-order harmonics obtained by irradiating short-pulse laser light to a nonlinear medium; a spectrometer comprising a grating structured to diffract the coherent light and an image sensor structured to measure an image of light diffracted by the grating; a first double slit having a pair of apertures arranged with an interval in a first direction, which are arranged at a predetermined position between the coherent light source and an incident slit of the spectrometer, in a first state; a second double slit having a pair of apertures that are a replica of the first double slit, which are arranged at the predetermined position as a replacement of the first double slit in a state in which one from among the pair of apertures holds a sample, in a second state; and a calculation processing device structured to calculate optical constants of the sample based on a first interference image formed on the image sensor due to the harmonics of the coherent light in the first state and a second interference image formed on the image sensor due to the harmonics in the second state, wherein the optical constant measurement apparatus is modeled using parameters, wherein the calculation processing device calculates an intensity distribution formed on the image sensor for each of the first state and the second state using a one-dimensional Fresnel diffraction expression based on the model of the optical constant measurement apparatus, and wherein the parameters of the model and the optical constants of the sample are measured such that the intensity distributions thus calculated approach the first interference image and the second interference image.