An image processing apparatus configured to perform image processing on a specimen image obtained by photographing a surface of a specimen of a woven or knitted fabric made of one or more synthetic fiber yarns, wherein the image processing apparatus executes a texture or stitch reduction process of making a texture or stitch pattern of the woven or knitted fabric less noticeable on the specimen image, wherein assuming that the number of repetitions of a density or brightness wave per a predetermined length with respect to a direction parallel to the surface of the specimen is N, a fluctuation in density or brightness on the specimen image is regarded as a composite waveform of a plurality of density or brightness waveforms of which numbers of repetitions N of the density or brightness wave are different from one another, and the texture or stitch reduction process comprises: a spectrum analysis process of performing a two-dimensional Fourier transform on the specimen image, thereby decomposing the fluctuation in density or brightness on the specimen image with respect to the direction parallel to the surface of the specimen into the plurality of density or brightness waveforms of which numbers of repetitions N of the density or brightness wave are different from one another, to obtain a density or brightness spectral distribution; a texture or stitch removal process of removing one or more density or brightness waveforms resulting from the texture or stitch pattern of the woven or knitted fabric from the density or brightness spectral distribution obtained in the spectrum analysis process; and an inverse transform process of performing a two-dimensional inverse Fourier transform on the density or brightness spectral distribution after the texture or stitch removal process, thereby obtaining the specimen image from which the texture or stitch pattern has been removed characterized in that in the texture or stitch removal process, assuming that the number of repetitions of the density or brightness wave of a waveform caused by the texture or stitch pattern with respect to a predetermined direction is defined as N1, all the density or brightness waveforms each of which number of repetitions of the density or brightness wave is not less than N1 are removed from the density or brightness spectral distribution. a spectrum analysis process of performing a two-dimensional Fourier transform on the specimen image, thereby decomposing the fluctuation in density or brightness on the specimen image with respect to the direction parallel to the surface of the specimen into the plurality of density or brightness waveforms of which numbers of repetitions N of the density or brightness wave are different from one another, to obtain a density or brightness spectral distribution; a texture or stitch removal process of removing one or more density or brightness waveforms resulting from the texture or stitch pattern of the woven or knitted fabric from the density or brightness spectral distribution obtained in the spectrum analysis process; and an inverse transform process of performing a two-dimensional inverse Fourier transform on the density or brightness spectral distribution after the texture or stitch removal process, thereby obtaining the specimen image from which the texture or stitch pattern has been removed